Extraction of dibenzothiophene from dodecane using ionic liquids

Extraction of dibenzothiophene from dodecane using ionic liquids

Fuel Processing Technology 93 (2012) 85–89 Contents lists available at SciVerse ScienceDirect Fuel Processing Technology journal homepage: www.elsev...

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Fuel Processing Technology 93 (2012) 85–89

Contents lists available at SciVerse ScienceDirect

Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Extraction of dibenzothiophene from dodecane using ionic liquids Cecilia Devi Wilfred a,⁎, Chong Fai Kiat a, Zakaria Man b, M. Azmi Bustam b, M. Ibrahim M. Mutalib b, Chan Zhe Phak c a b c

Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia PETRONAS Research Sdn Bhd, Lot 3288–3289, Off Jalan Ayer Itam, Kawasan Institusi Bangi, 43000 Kajang, Selangor, Malaysia

a r t i c l e

i n f o

Article history: Received 27 December 2010 Received in revised form 16 February 2011 Accepted 30 September 2011 Available online 1 November 2011 Keywords: Desulfurization Ionic liquids Extraction Dibenzothiophene

a b s t r a c t The effect of ionic liquid loading, extraction temperature, and extraction time in the removal of dibenzothiophene from dodecane were investigated. Eighteen (18) ionic liquids were screened for its dibenzothiophene extraction ability. Imidazolium based ionic liquids with thiocyanate, dicyanamide and octylsulfate anions exhibited the highest extraction capabilities with 66.1%, 66.1%, and 63.6% of extraction efficiency respectively. Tributylmethylammonium methylcarbonate ionic liquid gave 61.9% extraction efficiency, which showed that π–π interaction between aromatic rings of sulfur compound and ionic liquid (IL) was not be the main extraction mechanism. A trend between specific volume and desulfurization efficiency of ILs was put forward, enabling researchers to predict ILs' desulfurization efficiency from its specific volume. It was also found that [C4mim][SCN] can be reused in extraction without regeneration with considerable extraction efficiency of 41.9%. Huge saving on energy can be achieved if we make use of this IL behavior in process design, instead of regenerating IL after every time of extraction. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Fuels containing sulfur compounds generate sulfur oxide (SOx) in the combustion process, leading to severe environmental threat and serious diseases of human respiratory system. Consequently, the environmental regulations on sulfur limits are increasingly stringent. The European Union had proposed to reduce the sulfur content in diesel to a maximum 50 ppm in 2005 and below 10 ppm in 2009. This resulted in unavailability of sweet crude (low sulfur crude) in quantities required to meet the demand for low sulfur fuel, as world refining industry lacked the capacity to remove the required sulfur amounts from sour crude (high sulfur crude) and sour crude made up perhaps three quarters of world crude supply. Conventionally, sulfur compounds such as thiols (R\SH), thioethers (R\S\R), and disulfides (R\S\S\R) are removed efficiently in oil refinery through hydrodesulfurization (HDS) or hydrotreating unit. Nonetheless, aromatic sulfur compounds such as thiophenes, benzothiophenes, dibenzothiophenes, and their alkylated derivatives have great resistance to HDS making the attainment of ultra low-sulfur fuels a very expensive process [1–3]. Hydrogenation of those aromatic sulfur compounds requires increased energy, increased hydrogen consumption, and improved reactivity and selectivity of the catalyst.

⁎ Corresponding author. Tel.: + 60 5 3687635; fax: + 60 5 3656176. E-mail address: [email protected] (C.D. Wilfred). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.09.018

A study on typical Co–Mo catalyst showed that either four times more active catalysts or an increase of 38 °C in reaction temperature were needed to meet the required sulfur reduction of diesel products from 500 ppm to 50 ppm [4]. In addition, the side reactions induced such as hydrogenation of aromatics and olefins will result in decrease of octane number of the fuels. Therefore, alternative or complementary sulfur removal methods are needed to efficiently remove aromatic sulfur from oil. Oxidative desulfurization [5–6], adsorption desulfurization [7], biodesulfurization [8], and solvent extraction [5] are the methods widely studied by researchers. For solvent extraction method, volatility of most solvents may lead to solvent loss and toxicity concern. This problem is solved by ionic liquid as a new type of solvent with negligible vapor pressure. In contrast to normal liquid, ionic liquid is composed entirely from ions instead of molecules, which are bound mainly by ionic bond instead of van de Waals force. It strongly resembles ionic melt that may be produced by heating metallic salts. Ionic liquids are non-volatile, have high thermal stability and have recently gain interest in new solvent applications such as electrochemistry [9], separation [10], synthesis [11] and catalysis [11–12]. The physical properties such as density, viscosity, melting point, and hydrophobicity of ionic liquid can be tuned by careful choice of anion and cation. In this work, effect of mass ratio between ILs and dodecane (model oil), extraction temperature, and extraction time on dibenzothiophene (DBT) removal efficiency were investigated to determine a suitable set of extraction parameters. A total of eighteen (18) different ILs were used to extract DBT from dodecane respectively, and their extraction

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efficiency was correlated with their specific volume. The reusability of IL without regeneration was also investigated. Table 1 shows the list of ILs used in this work.

2.3. Preparation of DBT in dodecane Dodecane containing around 5 wt.% dibenzothiophene (DBT) was prepared by dissolving solid dibenzothiophene in n-dodecane. 5 wt.% of DBT in n-dodecane is equivalent to 8701 ppm of sulfur in n-dodecane.

2. Experimental 2.1. Materials

2.4. Effect of IL:dodecane mass ratio on DBT extraction

n-Dodecane and methanol were purchased from Merck whereas, dibenzothiophene was bought from Acros. [C4mim][SCN] was purchased from Sigma Aldrich and all the rest of the commercial ionic liquids were bought from Merck. [N4441][CH3CO3] was synthesized in the laboratory using literature methods [13]. All the ILs were dried in vacuum oven at 70 °C prior to being used with its water content measured.

[C4mim][SCN] and dodecane with 5 wt.% of DBT were mixed at 1:1 mass ratio. The mixture was heated in an oil bath at 30 °C with a 400 rpm stirring for 30 min. It was then left in the 30 °C oil bath to settle for 10 min. Samples from dodecane layer were taken out and analyzed for sulfur content. The same procedure was repeated for different IL:dodecane mixing ratio (2:1, 1:1, 1:5, and 1:10). Shimadzu GC-2010 gas chromatography (SGE BP1 capillary column, 30 m × 0.25 mm, 0.25 μm film thickness) with flame ionization detector was used to determine the concentration of DBT remained in the dodecane layer before and after desulfurization. The carrier gas was nitrogen, with a column flow rate of 1.39 ml·min− 1. The injector temperature was held at 593.15 K, and temperature in detectors was fixed at 603.15 K. The injection volume was 1 μl with a split ratio of 1:50. The oven temperature was from 323.15 K to 373.15 K with a ramp of 5 K·min− 1, to 593.15 K at 20 K·min− 1.

2.2. Ionic liquids synthesis and characterization [N4441][CH3CO3]: Tributylamine (9.27 g, 0.05 mol), dimethyl carbonate (4.50 g, 0.05 mol) and methanol (10 cm 3) were placed in a microwave quartz reaction tube, sealed and heated to 130 °C. The internal pressure and temperature of the reaction vessel were monitored. The resulting IL was sampled and analyzed by 1H NMR spectroscopy using D2O as solvent. The chemical shifts recorded are: δ 0.87–0.98 (t, 9H), 1.26–1.35 (m, 6H), 1.58–1.66 (m, 6H), 2.90 (s, 3H), 3.14– 3.18 (t, 6H), 3.28 (s, 3H).

2.5. Effect of extraction temperature on DBT extraction The experiment procedure for the above was repeated by changing the temperature in 10 °C increments.

Table 1 Ionic liquids used in this work.

2.6. Effect of Extraction Time on DBT Extraction

Ionic liquids

Abbreviation

Room temperature density (g/cm3)

1-butyl-3-methylimidazolium octylsulfate 1-butyl-3-methylimidazolium hydrogensulfate 1-butyl-3-methylimidazolium thiocyanate 1-butyl-3-methylimidazolium acetate

[C4mim] [C8H17SO4] [C4mim] [HSO4] [C4mim][SCN]

1.07

1-butyl-3-methylimidazolium trifluoromethanesulfonate 1-butyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium tris (pentafluoroethyl)trifluorophosphate 1-butyl-3-methylimidazolium methylsulfate 1-butyl-3-methylimidazolium dicyanamide 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide 1-butyl-3-methylimidazolium chloride 1-ethyl-3-methylimidazolium p-toluenesulfonate 1-ethyl-3-methylimidazolium diethylphosphate 1-ethyl-3-methylimidazolium tetrafluoroborate 1-hexyl-3-methylimidazolium tetrafluoroborate 1-hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide Tributylmethylammonium methylcarbonate

The experiment procedure for the above was repeated by changing the duration of the extraction (5 min, 10 min, 20 min, 30 min, and 60 min).

1.28 1.07

2.7. ILs desulfurization efficiency screening

[C4mim] [CH3CO2] [C4mim][OTf]

1.06

[C4mim][BF4]

1.21

[C4mim][PF6]

1.38

[C4mim][PF3 (CF2CF3)3] [C4mim] [CH3SO4] [C4mim][N (CN)2] [C4mim][NTf2]

1.63

1.44

The results of the optimization were applied to various ILs in desulfurization efficiency screening. Each IL was mixed with dodecane at 1:1 mass ratio. The mixture was heated in an oil bath at 30 °C with a 400 rpm stirring for 30 min. The mixture was then left in the 30 °C oil bath to settle for 10 min before the oil layer and the ionic liquid layer were sampled out for analysis. In order to measure DBT concentration in IL phase and dodecane phase, Agilant 1100 Series High Performance Liquid Chromatography equipped with diode array detector (DAD) and refractive index detector (RID) was used. Methanol/water (9:1) was used as the mobile phase in Zorbax SB-C18 column (4.6 mm× 150 mm, 5 μm) with a flow rate of 1 ml/min. Sample was injected into the HPLC directly at 1 μl without dilution. The UV wavelength used was 310 nm.

[C4mim][Cl] [C2mim][TOS]

Solid 1.23

[C2mim][DEP]

1.15

[C2mim][BF4]

1.34

[C6mim][BF4]

1.14

[C6mim][NTf2]

1.37

[N4441] [CH3CO3]

Solid

1.30

1.21 1.06

Density values are obtained from suppliers' website, room temperature = 20–25 °C.

2.8. IL reusability without regeneration [C4mim][SCN] and dodecane were mixed at 1:1 mass ratio. The mixture was heated in an oil bath at 30 °C with a 400 rpm stirring for 30 min. The mixture was then left in the 30 °C oil bath to settle for 10 min before the dodecane and the ionic liquid layer were sampled out from analysis. The separated ionic liquid layer was reused for three times using the same parameters. GC–FID was used to determine the concentration of DBT remained in the dodecane layer before and after desulfurization.

C.D. Wilfred et al. / Fuel Processing Technology 93 (2012) 85–89

Desulfurization Efficiency, %

80.0

to loss of n-dodecane into gas phase at higher temperature. The stable trend showed that the desulfurization efficiency of ILs was not sensitive to extraction temperature. For the IL desulfurization efficiency screening, extraction temperature at 30 °C was suitable because IL showed highest desulfurization efficiency at this point and it was close to room temperature. Other published work also showed the same trend [18]. There were also works which showed a stable trend with slight increase of desulfurization efficiency towards higher extraction temperature [16,19].

70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0.0

87

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

3.3. Effect of extraction time on DBT extraction

IL: Oil Mixing ratio, g/g Fig. 1. Desulfurization efficiency of [C4mim][SCN] at different IL:Oil mass ratio.

3. Results and discussion 3.1. Effect of IL:dodecane mass ratio on DBT extraction Fig. 1 shows the desulfurization efficiency at different mixing ratio for [C4mim][SCN]. Desulfurization efficiency here is defined by the following equation: Desulfurization efficiency ¼

½DBT  initial−½DBT  final  100% ½DBT  initial

In this extraction experiment, no presence of IL was detected by HPLC in the organic layer. The ionic liquid had its desulfurization efficiency increased when its mass ratio was increased. However, the increase of desulfurization efficiency was slower at high IL:Oil mass ratio. Therefore, it was obviously less efficient if we tried to increase desulfurization efficiency by increasing IL:Oil mass ratio. Multi-steps extraction should be considered to remove more sulfur instead. For IL desulfurization efficiency screening, IL:Oil mass ratio of 1 was suitable because IL mass ratio beyond this was deemed uneconomic for practical application in the industry. Similar experimental results were published by other researchers, where increasing the IL:Oil mass ratio would result in the increase of desulfurization efficiency [14–15]. Some researchers also found a point where the IL desulfurization efficiency increased only marginally with further increase of IL:Oil mass ratio [16–17]. 3.2. Effect of extraction temperature on DBT extraction Fig. 2 shows the effect of temperature on model oil desulfurization efficiency of [C4mim][SCN]. The desulfurization efficiency was quite stable from 30 °C to 80 °C, with a slight decrease towards 80 °C. The slight decrease of desulfurization efficiency was most probably due

Fig. 3 shows the desulfurization efficiency of [C4mim][SCN], [C4mim][OcSO4], and [C4mim][CH3CO2] at different extraction time. Viscous ILs took less than 30 min to achieve liquid–liquid equilibrium, as shown by [C4mim][OcSO4], and [C4mim][CH3CO2]. This shows that the desulfurization efficiency of the ILs was at its optimum at 30 min of extraction time. Similar results were obtained by other researchers, where the required extraction time ranged from 10 min to 25 min [15,17,19]. 3.4. IL desulfurization efficiency screening The desulfurization efficiency of various ILs is shown in Fig. 4. [C4mim][SCN], [C4mim][N(CN)2], [C4mim][C8H17SO4], and [N4441] [CH3CO3] were the top four ILs with 66.1%, 66.1%, 63.6%, and 61.9% of desulfurization efficiency. Although [C4mim][C8H17SO4] showed good desulfurization efficiency, large amount of n-dodecane from model oil dissolved in it, causing undesirable loss of hydrocarbon. The desulfurization efficiency of [C4mim][N(CN)2] was reported at 68.9% in literature [18] under similar experimental conditions. ILs with better physical extraction performance were disclosed in another paper [13], however, fair comparison couldn't be made now as the extraction experiments were conducted using volume ratio rather than mass ratio. Throughout the experiment, the mass ratio of DBT, dodecane, and IL was fixed for the purpose of desulfurization efficiency screening. Tie line study is in progress for selected system. In most of the earlier works [16,20], π–π interaction was thought to be the main interaction between ILs and aromatic sulfur species, owing to the good desulfurization efficiency of ILs containing aromatic rings such as imidazolium and pyridinium. However, the good desulfurization efficiency of [N4441][CH3CO3] which has no aromatic ring in its structure breaks this notion. The electrostatic attraction between IL and aromatic sulfur species seems to better explain the interaction between IL and sulfur compounds here [21]. Another example was [C4mpyrr][NTf2] which was disclosed in literature [13]. It has a very close desulfurization efficiency with [C4mim][NTf2] even though without any aromatic structure.

70.0

Desulfurization Efficiency, %

Desulfurization Efficiency, %

70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 30

65.0

60.0

55.0

50.0

45.0 40

50

60

70

80

Temperature,oC Fig. 2. Desulfurization efficiency of [C4mim][SCN] at different extraction temperature.

0

5

10

15

20

25

30

35

40

45

50

55

60

Mixing Time, minutes Fig. 3. Desulfurization efficiency of [C4mim][SCN] at different extraction time.

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C.D. Wilfred et al. / Fuel Processing Technology 93 (2012) 85–89

Desulfurization Efficiency, %

Excludes ILs with > 4 electronegative atoms

70 60 50 40 30 20 10 0

1

2

3

4

5

1. [C4mim][SCN] 2. [C4mim][N(CN)2] 3. [C4mim][C8H17SO4] 4. [N4441][CH3CO3]* 5. [C4mim][CH3CO2] 6. [C2mim][DEP]

6

7

8

9

7. [C6mim][BF4] 8. [C6mim][NTf2] 9. [C4mim][CH3SO4] 10. [C4mim][OTf] 11. [C4mim][NTf2] 12. [C4mim][PF3(CF2CF3)3]

13. [C4mim][Cl]* 14. [C4mim][BF4] 15. [C4mim][PF6] 16. [C2mim][TOS] 17. [C2mim][BF4] 18. [C4mim][HSO4]

o

Fig. 4. Desulfurization efficiency of various ionic liquids.

Partition coefficient (K N) was calculated from DBT concentration in oil phase and IL phase after extraction. The DBT concentration in oil phase and IL phase was directly determined using HPLC method. Partition coefficient values of the ILs with good DBT removal are shown in Table 2. [N4441][CH3CO3] was solid at room temperature, therefore, direct determination of the DBT content in IL phase after extraction was impossible. Using the results from ILs desulfurization efficiency screening, we found a trend between specific volume and desulfurization efficiency of ILs, as shown in Fig. 5. Specific volume is defined by the following equation: Specific volume ¼

1 Density

ILs with higher specific volume showed higher desulfurization efficiency. It was because ILs with higher specific volume had more free Table 2 Desulfurization efficiency and partition coefficient of various ionic liquids. No

Ionic liquids

Water (wt.%)

DBT in oil phase after extraction (wt.%)

DBT in IL phase after extraction (wt.%)

Partition coefficient, KN

Desulfurization efficiency (%)

1

[C4mim] [SCN] [C4mim][N (CN)2] [C4mim] [C8H17SO4] [N4441] [CH3CO3]a [C4mim] [CH3CO2] [C2mim] [DEP] [C6mim] [BF4] [C6mim] [NTf2] [C4mim] [CH3SO4] [C4mim] [OTf]

0.31

1.61

3.24

2.01

66.1

0.07

1.63

3.11

1.90

66.1

0.18

1.74

2.85

1.64

63.6



1.83





61.9

0.88

2.07

2.70

1.30

56.5

0.08

2.17

2.70

1.24

53.9

0.03

2.19

2.72

1.24

53.5

0.02

2.29

2.50

1.09

51.6

0.23

2.37

2.52

1.06

50.2

0.08

2.63

2.14

0.81

44.8

3 4 5 6 7 8 9 10

70 60 50 40 30 y = 282.94x - 200.26 R² = 0.7966

20 10 0 0.7

0.75

0.8

0.85

0.9

0.95

1

10 11 12 13 14 15 16 17 18

Mixing temp = 30 C, Mixing time = 30mins, IL:Oil weight ratio = 1:1 (5g each), stirring speed = 400rpm, mixing vessel = 50ml screw-cap glass bottle. * Mixing temp = 80ºC for non room temperature IL

2

80

Specific Volume, cm3/g Fig. 5. Specific volume vs desulfurization efficiency of ionic liquids.

space between ions to accommodate DBT molecules. Besides that, ILs with lower density might also result in a higher chance for the cations to interact with the electron-rich sulfur compounds [22]. This trend enables us to easily predict the desulfurization performance of ILs from its specific volume. The ILs' sequence according to its specific volume was [C4mim][CH3CO2] N [C4mim][N(CN)2] N [C4mim] [SCN] N [C4mim][C8H17SO4] N [C6mim][BF4] N [C2mim][DEP] N [C4mim] [CH3SO4] N [C4mim][BF4] N [C2mim][TOS] N [C4mim][HSO4] N [C2mim] [BF4]. However, a few exceptions were observed on ILs with anions containing more than 4 highly electronegative atoms, such as [NTf2] −, [OTf] −, [PF3(CF2CF3)3] −, and [PF6] −. Although they were low in specific volume, these ILs showed moderate desulfurization efficiency because they contain large polar surface to interact with DBT. 3.5. IL reusability without regeneration Fig. 6 shows the desulfurization efficiency of [C4mim][SCN] which was reused for three times without regeneration. It showed that the used IL was able to extract DBT from model oil even without regeneration, however, at a lower efficiency. The desulfurization efficiency of reused IL could also be calculated from its partition coefficient as shown in Figs. 6 and 7. Huge saving on energy can be achieved if we make use of this IL behavior in process design, instead of regenerating IL after every time of extraction. 4. Conclusion Among the 18 ILs screened for its desulfurization efficiency, [C4mim][SCN], [C4mim][N(CN)2], [C4mim][C8H17SO4], and [N4441] [CH3CO3] were the top four ILs with 66.1%, 66.1%, 63.6%, and 61.9% of desulfurization efficiency. The good desulfurization efficiency of [N4441][CH3CO3] showed that π–π interaction between aromatic rings of sulfur compound and IL might not be the main extraction

80.0

Mixing temp = 30 °C, Mixing time = 30 min., IL:Oil weight ratio = 1:1 (5 g each), stirring speed = 400 rpm, mixing vessel = 50 ml screw-cap glass bottle. a Mixing temp = 80 °C for non room temperature IL.

Desulfurization Efficiency, %

Desulfurization Efficiency, %

80

70.0

[C4mim][SCN]

60.0

Calculated from Partition Coefficient

50.0 40.0 30.0 20.0 10.0 0.0 1st Time

2nd Time

3rd Time

4th Time

Fig. 6. Desulfurization efficiency of [C4mim][SCN] which was reused for three times without regeneration.

C.D. Wilfred et al. / Fuel Processing Technology 93 (2012) 85–89

Desulfurization Efficiency, %

80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0

20th

18th

19th

17th

15th

16th

14th

12th

13th

11th

9th

10th

8th

7th

6th

5th

4th

3rd

1st

2nd

0.0

Fig. 7. Desulfurization efficiency of reused [C4mim][SCN] calculated from partition coefficient.

mechanism. A trend between specific volume and desulfurization efficiency of ILs was discovered, enabling researchers to predict IL's desulfurization efficiency from its specific volume. It was found that IL can be reused in extraction without regeneration with considerable desulfurization efficiency. Huge saving on energy can be achieved if we make use of this IL behavior in process design, instead of regenerating IL after every time of extraction. Acknowledgment The authors would like to thank Universiti Teknologi PETRONAS for the facilities provided in this work and PETRONAS sponsorship for Chan Zhe Phak. Reference [1] L. Alonso, A. Arce, M. Francisco, A. Soto, (Liquid + liquid) equilibria of [C8mim] [NTf2] ionic liquid with a sulfur-component and hydrocarbons, The Journal of Chemical Thermodynamics 40 (2008) 265–270. [2] L. Alonso, A. Arce, M. Francisco, A. Soto, Thiophene separation from aliphatic hydrocarbons using the 1-ethyl-3-methylimidazolium ethylsulfate ionic liquid, Fluid Phase Equilibria 270 (2008) 97–102. [3] L. Alonso, A. Arce, M. Francisco, A. Soto, Solvent extraction of thiophene from n-alkanes (C7, C12, and C16) using the ionic liquid [C8mim][BF4], Journal of Chemical Thermodynamics 40 (2008) 966–972.

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