High-Efficient Oxidation–Extraction Desulfurization Process by Ionic

Article pubs.acs.org/EF

High-Efficient Oxidation−Extraction Desulfurization Process by Ionic Liquid 1‑Butyl-3-methyl-imidazolium Trifluoroacetic Acid Dawei Fang,† Qiang Wang,‡ Yu Liu,† Lixin Xia,*,† and Shuliang Zang*,†,‡ †

Key Laboratory of Rare and Scattered Elements, College of Chemistry, Liaoning University, Shenyang, Liaoning 110036, People’s Republic of China ‡ School of Chemistry and Materials Science, Liaoning Shihua University, Fushun, Liaoning 113001, People’s Republic of China ABSTRACT: A Brønsted acid ionic liquid (IL), [C4mim]TFA (1-butyl-3-methyl-imidazolium trifluoroacetic acid), was synthesized and used as an extractant and catalyst for oxidation desulfurization (ODS) of thiophene from model oil with hydrogen peroxide as the oxidant. The major factors of the reaction were investigated in detail. The effect of different types of ILs on oxidative and extractive desulfurization capacities of model oil was tested, which proved TFA− > HSO4− > COO− > AlCl4− > AcO−. The reaction heat caused by H2O2 dissolved in IL was determined by a calorimeter.

1. INTRODUCTION The sulfocompound existing in fuel will transform into SOx through combustion, which causes environmental pollution.1−3 To minimize the negative effects of the exhaust gas SOx, which are emitted by the vehicle to the environment, the sulfur level in transportation fuels must be eliminated. Removal of sulfur from fuels has been an important and challenging issue worldwide. The U.S.A. and many countries in Europe have taken the lead in implementing more stringent regulations for sulfur content in fuels.4 Ultra-low-sulfur fuels are being called for worldwide in the coming years. It is a challenging task that worldwide researchers have been dedicated to using desulfurization processes in recent decades. At present, fuel desulfurization technology mainly contains two kinds, hydrodesulfurization (HDS) and non-hydrodesulfurization (NHDS). HDS is a conventional method to remove sulfur compounds in industrial purposes.5−7 Although it developed rapidly and had a good desulfurization effect, olefins were saturated, the octane rating was low, and hydrogen consumption is large in the process. In addition, HDS required a high temperature and high pressure. Therefore, the huge capital investment and the high operating costs are the main drawbacks of HDS.8 The most important defect is that aromatic heterocyclic sulfur compounds, such as thiophene and its derivatives, are difficult to remove by HDS. To solve the above problems in desulfurization processes, many new desulfurization methods without hydrogen have been found and researched by researchers, such as bioprocesses,9 adsorption,10,11 extraction,4,12 and oxidation.13,14 The study found that, if these desulfurization methods cooperate for use, the desulfurization effect is better. Among these methods, oxidation desulfurization (ODS) with its moderate reaction conditions has attracted more attention as a promising method to remove sulur-containing compounds.15 ODS has a excellent desulfurization effect, but huge amounts of volatile and flammable organic extractant used in the process cause further environmental and safety problems. In recent years, ionic liquids (ILs) have attracted considerable attention and been used extensively, owing to © XXXX American Chemical Society

their special physical and chemical properties, such as low vapor pressure, nonflammability, good thermal stability, easy recycling, high conductivity, strong dissolving capacity, etc.16−18 Extractive desulfurization using ILs has been reported in the literature because ILs show good extracting ability for aromatic sulfur-containing compounds and are immiscible with aliphatics, such as fuels.19−21 Especially, ODS combined with H2O2 as the oxidant and ILs as the extractant and catalyzer was developed rapidly.22,23 ODS/extraction involves two steps in the reaction process. First, oxidizing sulfur compounds, such as thiophene and its derivative, were transformed into sulfones and sulfoxides by the oxidant. In a second step, these oxidized products are removed by selective extraction with polar solvents.24,25 Now, it has been investigated extensively that the ODS/extraction setting of H2O2 as the oxidant and ILs as the extraction removes sulfur compounds.26−28 In previous studies, ILs, particularly acidic ILs, have been proven to have higher extraction ratios and greater selectivity than traditional solvents, owing to their unique characteristics. In this paper, a Brønsted acid IL, [C4mim]TFA (1-butyl-3methyl-imidazolium trifluoroacetic acid), was synthesized and used as an extractant and catalyst for ODS of thiophene in noctane in combination with hydrogen peroxide as the oxidant. The major factors of the reaction, such as the effects of the amount of IL and oxidant, the reaction time and temperature on ODS/extraction efficiency, and the recycling of IL, were investigated in detail. The reaction heat caused by H2O2 was determined by a calorimeter.

2. EXPERIMENTAL SECTION 2.1. Preparation of [C4mim]TFA. [C4mim]TFA was prepared by a two-step process according to a literature procedure.29 The intermediate [C4mim]Br (1-butyl-3-methyl-imidazolium bromide) was prepared by the published procedure described in the literature.30 1-Methylimidazole and 1-chlorobutane were placed in a flask in the same molar ratio. The mixture was stirred at 70 °C for 8 h. When the Received: July 28, 2014 Revised: September 10, 2014

A

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reaction completed, the resulting liquid was washed with ethyl acetate 3 times and purified through freezing recrystallization and vacuum filtering. The residue was dried in vacuum to remove impurities. Then, the intermediate [C4mim]Br was obtained as a white solid. Equimolar amounts of trifluoroacetic acid were added dropwise to a solution of intermediate product and water while stirring at 0 °C. After completion of addition, the reaction mixture was stirred at room temperature for an additional period of 2 h. The residual reactant, HBr, and solvent were removed by a rotary evaporator. The product was dried in vacuum to give a yellow viscous liquid. The obtained ILs were characterized and verified by 1H nuclear magnetic resonance (NMR) and 19F NMR. 2.2. Desulfurization Procedure. A total of 0.5 mL of thiophene was dissolved in 1000 mL of n-octane to form the model oil, where the sulfocompound content was 202.544 μg mL−1. The ODS/extraction reactions were carried out in a flask immersed in a water bath with a temperature-controlling system. A certain amount of model oil, H2O2 (30%), and IL was mixed and stirred vigorously for some time at a certain temperature. After the reaction, the upper phase (model oil) was separated and analyzed for sulfur content by a microcoulometric detector (type WK-2D), produced by Jiangsu Analysis Intrument Co., Ltd. 2.3. Regeneration of Used IL. The possibility of regeneration of the ILs was also investigated to understand better the advantage of using ILs for ODS. After the first reaction, the upper layer (the oil phase) was taken off by a separatory funnel as completely as possible from the IL. The used ILs were evaporated at 80 °C by a rotary evaporator to remove hydrogen peroxide, water, and marginal model oil.

Figure 2. Sulfur content of different amounts of H2O2 in the presence of [C4mim]TFA.

desulfurization. The reactions were carried out with 20 mL of model oil and 0.5 mL of [C4mim]TFA at 60 °C for 60 min. From the results, with the increase of the amount of H2O2 from 0.125 to 1 mL, the sulfur content varied from 79.24 to 37.52 μg mL−1. The result indicated that the more oxidants used, the higher the extraction efficiency can be obtained. When the amount of H2O2 was 1 mL, the sulfur content decreased to 37.52 μg mL−1. 3.3. Effect of the Temperature on Thiophene Removal with [C4mim]TFA. Figure 3 shows the effect of the

3. RESULTS AND DISCUSSION 3.1. Effect of the Mass Ratio between IL and Model Oil on Thiophene Removal. The mass ratio between model oil and IL was an important factor upon extraction. A total of 0.5 mL of [C4mim]TFA was used as the extractant, and 0.25 mL of H2O2 was used as the oxidant, at 60 °C for 60 min. The results are shown in Figure 1. From the results, when the mass

Figure 3. Sulfur content of different temperatures.

temperature on thiophene removal with [C4mim]TFA. It can be seen that, with the temperature rising, the sulfur content decreased. As the system temperature increased from 50 to 80 °C, the sulfur content varied from 65.361 to 53.031 μg mL−1. The reason for the above experiments was that the viscosity of IL decreased and the catalytic performance increased with increasing the temperature, which was beneficial for the removal of thiophene. However, the decomposition of H2O2 will accelerate in higher temperatures. Therefore, the excessive high temperature would lead to low desulfurization efficiency. 3.4. Effect of the Reaction Time on Thiophene Removal with [C4mim]TFA. Figure 4 reveals the effect of

Figure 1. Sulfur content of different mass ratios between model oil and IL in the presence of H2O2.

ratio between model oil and IL decreased from 80 to 20, the sulfur content varied from 86.47 to 60.60 μg mL−1. The result indicated that the more IL used, the higher the extraction efficiency can be obtained. When the mass ratio was 20:1, the sulfur content decreased to about 60 μg mL−1. 3.2. Effect of the Amount of Oxidants on Thiophene Removal. Figure 2 reveals the effect of the amount of H2O2 on B

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When the anion was HSO4−, the order of the desulfurization capacity of different cationtypes was [C5mim]+ > [C3Py]+ > [C4mim]+ > [C3mim]+ > [HSO3−C3 mim]+ > [HSO4−C3EA]+ > [HSO3−C5Py]+. It shows that ILs with cation [Cnmim]+ (including [C2mim], [C3mim], [C4mim], and [C5mim]) have better extraction capability than those with [HSO3−C3mim]+, [HSO4−C3EA]+, and [HSO3−C5Py]+ as the cation and the branch length of cations plays a role in the desulfurization process. 3.7. Optimization of the ODS/Extraction Parameters on Thiophene Removal by the Orthogonal Experiment. The ODS/extraction of thiophene in n-octane with IL [C4mim]TFA and H2O2 was optimized by the orthogonal experiment. An orthogonal L9(34) test design was used to find out the optimization of the ODS/extraction conditions. The factors and levels for the orthogonal L9(34) test are presented in Table 2a. The results of the orthogonal test and the extreme difference analysis are presented in Table 2b. From the statistics given in Table 2, it can be seen that the optimal combination for ODS/extraction of thiophene, A1B3C3D3, was acquired from the orthogonal L9(34) test. In other words, the optimal conditions for ODS/extraction of thiophene are the oxidation temperature at 80 °C, oxidation time of 120 min, volume ratio of model oil/IL of 25:1, and amount of oxidants of 2 mL with a result of almost zero sulfur content. The ninth result in Table 2 also expressed the ideal result with the sulfur content, 7.320 μg mL−1, which achieved the Euro V standard. The influence of factors on the desulfurization of thiophene decreases in the order: B > C > A > D, according to the extreme difference analysis. The factors A and D, volume ratio of model oil/IL and oxidation time, were found to be the last two determinants of the desulfurization. Therefore, the value of Vmodel oil/VIL can be more than 25:1, and the oxidation time can be less than 120 min, in practical operations to reduce the cost of ODS/extraction. 3.8. Mechanism of the Desulfuration Process. The sulfocompounds existing in fuel are mostly high polar compounds, which have similar solubility to ILs with high polarity. This provides the method of extraction from the oil phase. Also, an improved method is to make the sulfocompound higher in polarity. The S−C bond in thiophene has nearly no polarity, but the d orbit exists in the −S− atom, which can be oxidated. Therefore, thiophene could be easily oxidated to sulfone, which has similar polarity to ILs. Sulfone can dissolve in the ILs, owing to the enhancement of solubility

Figure 4. Sulfur content of different extraction times.

different reaction times in the system. From the results, It can be seen that, as time increased to 120 min, the sulfur content decreased to 59.139 μg mL−1 and the extraction equilibrium can be reached within 120 min. 3.5. Recycling of the Used [C4mim]TFA. After the first cycle, the model oil was separated by decantation from the IL phase. Fresh model oil and H2O2 were introduced to the next cycle under the same conditions. The used IL was directly used in the next cycle without any treatment. The sulfur content in model oil decreased to 36.279 μg mL−1 in the first repeating cycle. The sulfur content decreased to 50.011 μg mL−1 in the fifth repeating cycle, because the IL became saturated gradually and the extraction capability decreased by degree with increasing the repeating times. 3.6. Effect of Different ILs on Thiophene Removal. According to the literature, several ILs were synthesized.31−36 The effect of different types of ILs on oxidative and extractive desulfurization capacities of model oil is shown in Table 1. Under the same conditions, the mass ratio between model oil, IL, and H2O2 was 5:1:1, the reaction time was 60 min, and the temperature was 60 °C. Their oxidative and extractive desulfurization capabilities presented clear differences. With imidazolium as the cation, their desulfurization capability decreased in the following order: TFA− > HSO4− > COO− > AlCl4− > AcO−. It shows that, when the acidity of IL was much stronger, the catalytic and extractive capability was better.

Table 1. Sulfur Content Oxidized and Extracted by Several Different Kinds of ILs IL

volume ratio (Vmodel oil/VIL)

oxidants (mL)

temperature (°C)

time (min)

sulfur content (μg mL−1)

[C2mim]TFA [C3mim]TFA [C5mim]TFA [C3mim]HSO4 [C4mim]HSO4 [C5mim]HSO4 [C3Py]HSO4 [HSO3−C3mim]HSO4 [HSO4−TEA]HSO4 [HSO3−C3Py]HSO4 [C4mim]OOC [C4mim]OAc [C4mim]AlCl4

5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1

1 0.5 1 1 1 1 1 1 1 1 1 1 1

60 60 60 60 60 60 60 60 60 60 60 60 60

60 60 60 60 60 60 60 60 60 60 60 60 120

21.168 26.368 43.309 64.811 54.928 46.215 54.316 167.909 184.239 195.580 97.314 159.760 155.362

C

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Table 2. (a) Factors and Levels for the Orthogonal Test of [C4mim]TFA and (b) Results and Analysis of the Orthogonal Test (a) Factors and Levels for the Orthogonal Test of [C4mim]TFA A, volume ratio (Vmodel oil/ VIL)

entry 1 2 3 4 5 6 7 8 9 average of indicators

25:1 25:1 25:1 30:1 30:1 30:1 35:1 35:1 35:1 20.305 26.523 23.857 6.218 A1 3

k1 k2 k3

extreme difference optimization level order of factors

a

B, oxidants (mL)

C, temperature (°C)

D, time (min)

sulfur content (μg mL−1)

30 60 120 120 30 60 60 120 30 23.962 23.363 23.361 0.601 D3 4

43.700 17.214 a 34.501 20.865 24.204 28.670 35.581 7.320

0.5 60 1 70 2 80 0.5 70 1 80 2 60 0.5 80 1 60 2 70 35.624 34.495 24.553 19.678 10.508 16.512 25.116 17.983 B3 C3 1 2 (b) Results and Analysis of the Orthogonal Test

level

A, volume ratio (Vmodel oil/ VIL)

B, oxidants (mL)

C, temperature (°C)

D, time (min)

1 2 3

25:1 30:1 35:1

0.5 1 2

60 70 80

30 60 120

Cannot be detected.

caused by the −O− atom bond with the sulfocompound. In this case, if H2O2 was added to the system, the reaction proceeded more quickly. H2O2 will help oxidate thiophene to sulfone compounds. The process was in Figure 5.

The values of solution enthalpy of [C4mim]TFA with various molalities in water at 298.15 K are listed in Table 3. From Table 3, the solution processes of IL [C4mim]TFA are endothermic. Table 3. Values of Molar Solution Enthalpy of [C4mim]TFA at Different Molalities

Figure 5. Process of oxidation−extraction desulfurization by IL.

In Figure 5, thiophene was first extracted into the IL phase and then oxidated to sulfone compounds by IL and H2O2. The sulfone compounds were kept in the IL phase, owing to their similar polarity. When the equilibrium was broken, thiophene in the oil phase was continuously extracted into IL phase until the IL was saturated. 3.9. Theoretical Research of the IL Phase. The IL phase contains [C4mim]TFA, H2O2, and water. The IL is dissolvable in water; therefore, the desulfurization process is also a dissolvation process accompanied with the increase of H2O2. Because the industrial process has a strict rule dealing with oil, which is gasoline and diesel, the heat caused by dissolvation must be measured for safety concerns. According to our previous investigations,37−40 a calorimeter was used to determine the solution heat of the process.

m (mol kg−1)

ΔsHm (J mol−1)

0.00121508 0.00161587 0.00175079 0.00201151 0.00241865 0.00277579 0.00298730 0.00340000 0.00359167 0.00384960 0.00422619 0.00450000 0.00500000 0.00520317 0.00546032

−9498 −10229 −10318 −10591 −11017 −11089 −11233 −11727 −11843 −11973 −12262 −12418 −12632 −12849 −12880

Φ

L (kJ mol−1) −5.40 −6.13 −6.22 −6.49 −6.92 −6.99 −7.14 −7.63 −7.75 −7.88 −8.17 −8.32 −8.54 −8.75 −8.78

The molar solution enthalpy of [C4mim]TFA is expressed as ΔsHm = ΔsHm 0 + ϕL

(1)

where ΔsHm is the molar solution enthalpy at infinite dilution and ϕL is the apparent relative molar enthalpy. According to the Pitzer theory,41 we have 0

ϕ

L = 2(AH /3.6)ln(1 + 1.2I1/2) − 2RT 2 ′ + m2CMX ′ ) (mBMX

(2)

where I means ionic strength, R is the gas constant, and other symbols are defined by the following equations: D

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sulfocompound content could achieve almost zero. The effect of different types of ILs on oxidative and extractive desulfurization capacities was tested, which proved TFA− > HSO4− > COO− > AlCl4− > AcO−. The reaction heat caused by H2O2 dissolved in IL was determined by a calorimeter. The process is endothermic, and the molar solution enthalpy of [C4mim]TFA at infinite dilution is ΔsHm0 = −4.0963 kJ mol−1. This study will provide new method of desulfurization technology in industry, that is, ODS after existing nomal HDS. In this process, the residue sulfocompound content in oil is less than 10 μg mL−1, which satisfied the Euro V standard.

2β (1)L ⎛ ∂B ⎞ (0)L ′ = ⎜ MX ⎟ = βMX BMX + MX ⎝ ∂T ⎠P,I α 2I [1 − (1 + αI )exp(−αI1/2)]

(3)

⎛ ∂β (0) ⎞ (L)0 βMX = ⎜⎜ MX ⎟⎟ ⎝ ∂T ⎠P

(4)

⎛ ∂β (1) ⎞ (1)L βMX = ⎜⎜ MX ⎟⎟ ⎝ ∂T ⎠P

(5)

′ CMX ϕ

L CMX

ϕ 1 L = (νMνX)1/2 CMX 2

(6)

⎛ ∂C ϕ ⎞ = ⎜⎜ MX ⎟⎟ ⎝ ∂T ⎠P

(7)

β(0) MX

■

Corresponding Authors

*E-mail: davidfi[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

β(1) MX

where and show different kinds of reactions of short distance among ionics and CϕMX is the triple-ionic action term, which indicates the important interaction in higher molalities. (1) ϕ The values of Pitzer’s parameters, β(0) MX, βMX, and CMX, were obtained from the experimental data by the least-squares method. AH, the Debye−Hückel parameter of enthalpy, is 1986 J mol−1 at 298.150 K. From eqs 2 and 1, the working equation to determine Pitzer’s parameters was obtained38 Y=

ΔsHm − 2(AH /3.6)ln(1.2I1/2) 2RT 2

ACKNOWLEDGMENTS This project was financially supported by the National Key Technology Research and Development Program (2012BAF03B02), the National Natural Science Foundation of China (NSFC, 21373005, 21271095, and 21471073), the Science and Technology Bureau of Liaoning Province (201202084), the Education Bureau of Liaoning Province (LR2012001), the Shenyang Science and Technology Program (F12-265-4-00), and the Liaoning Provincial Department of Education Innovation Team Project (LT2012001).

(0)L = a0 − mβMX − my′

■

ϕ

(1)L L /2 βMX − m2CMX

(8)

y′ =

ΔsHm 0 2RT 2 1 [1 − (1 + 2I1/2)exp( −2I1/2)] 2I

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where Y is the extrapolate function, which can be obtained by experimental data. α0 and y′ in eq 8 are

a0 =

AUTHOR INFORMATION

(9) (10)

The value of ΔsHm was calculated according to eq 8. The values of the parameters are as follows: a0 = −0.00277 and −0.85097, ϕ L (1)L β(0)L MX = −0.14452, βMX = 49.768, CMX, standard deviation of fit s −4 = 0.6 × 10 , and correlation coefficient = 0.99. The molar solution enthalpy of [C4mim]TFA at infinite dilution was calculated from the parameter a0, and its value was ΔsHm0 = −4.0963 kJ mol−1. The process of dissolvation is endothermic, and the molar solution enthalpy at infinite dilution is ΔsHm0 = −4.0963 kJ mol−1. Therefore, in industry, the reactor should provide more heat to protect the damage from freezing, owing to the dissolvation. In another aspect, seeking new oxidation without water is an urgent task for the next step.

4. CONCLUSION In this paper, a Brønsted acid IL, [C4mim]TFA, was synthesized and used as an extractant and catalyst for ODS from model oil with hydrogen peroxide as the oxidant. The major factors of the reaction were investigated in detail, which verified the best optimistic conditions, that is, a mass ratio of oil/IL of 35:1 with 2 mL of H2O2 at 70 °C for over 30 min. The E

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F

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