Efficient and sustainable V-catalyzed oxidative desulfurization of fuels assisted by ionic liquids

Efficient and sustainable V-catalyzed oxidative desulfurization of fuels assisted by ionic liquids

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 46, Issue 9, September 2018 Online English edition of the Chinese language journal Cite this article a...

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JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 46, Issue 9, September 2018 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2018, 46(9), 11211129

RESEARCH PAPER

Efficient and sustainable V-catalyzed oxidative desulfurization of fuels assisted by ionic liquids Alessia Coletti1, Federica Sabuzi1, Barbara Floris2, Pierluca Galloni1,2,*, Valeria Conte1,* 1

Department of Chemical Science and Technologies, University of Rome Tor Vergata, via della Ricerca Scientifica snc, Rome 00133, Italy;

2

BT-InnoVaChem srl, Academic Spin-Off, c/o University of Rome Tor Vergata, via della Ricerca Scientifica snc, Rome 00133, Italy

Abstract:

Fuel desulfurization is an appealing topic for the chemical industry since severe environmental regulations regarding SO 2

emissions have been legislated in many countries. In order to reduce the amount of sulfur-containing compounds in fuels, responsible for high SOx emission levels, a green chemistry approach is compulsory. In this paper, vanadium salen and salophen complexes were used in the oxidation of a model aromatic sulfide, such as dibenzothiophene (DBT), in the presence of H 2O2 as green oxidant. The oxidative process was successfully coupled with the extraction of the oxidized compounds by ionic liquids. The system resulted highly selective for sulfide oxidation, showing poor reactivity toward the oxidation of alkenes and allowing a significant reduction of S content in a model benzine. To note, the use of microwave in place of standard heating allowed to obtain 98% of DBT oxidation and almost complete sulfur extraction in the model fuel in 1000 s. For these reasons, this system was considered an easy, rapid and clean process to achieve fuel desulfurization. Key words:

fuel desulfurization; V-catalysis; sustainability; ionic liquids; microwaves

Despite the exploration of sustainable and renewable energy is nowadays one of the most appealing research topic, fossil fuels and in particular oil still represent the most used energy sources. However, it is common knowledge that combustion of huge quantities of crude oil is among the major responsible for the increase in carbon dioxide as well as nitrogen and sulfur oxides (NOx and SOx) emissions in the atmosphere, thus raising the greenhouse effect and the amount of acid rains. Therefore, together with CO2 and NOx emissions control, sulfide removal from fuels is one of the most useful strategy to reduce the environmental impact related to the intensive use of energy from oil[1,2]. In fact, sulfur is one of the most abundant natural components of crude oil and it is still present into gasoline in the form of alkyl sulfur compounds and in diesel, which is particularly rich of aromatic organosulfur compounds. The main issue related to the presence of sulfur-containing compounds is their oxidation to SOx during combustion, which is responsible for air pollution. In order to reduce sulfur content from fuel, green chemistry contribution is essential. Currently, the most widely adopted method in industries is the hydrodesulfurization (HDS)[1,3]. According to this process, S-compounds react with H2 in the presence of a catalyst to

produce H2S, which is then removed. Although such approach is effective for the removal of aliphatic sulfur-containing compounds, the removal of heterocyclic molecules, such as thiophene (TS), benzothiophene (BT), dibenzothiophene (DBT) and their derivatives, usually being more than 50% of the sulfur content in diesel, is still a challenge. In fact, due to the steric hindrance of such substrates, the enforceability of this method is limited because harsh conditions are required, i.e. very high temperature (300‒400°C) and pressure (20–100 atm of H2) and also highly reactive catalysts. These features clearly significantly increase the costs of the process as well as the environmental impact. To overcome these problems, several alternative processes have been developed. Such approaches are commonly based on biodesulfurization, where a microbial system is used to desulfurize a broad range of organosulfur compounds in crude oil[1,4,5], selective adsorption on different materials such as carbon-based adsorbents[1,6], inorganic surfaces[1,7], molecular imprinted polymers[1,8,9], or metal-organic frameworks[10–12]. However, biodesulfurization is still not suitable for industrial applications because of the low rate of the process, while

Received: 14-Jun-2018; Revised: 06-Aug-2018. Foundation items: Supported by the University of Rome “Tor Vergata”, SUSCARE project. Corresponding author. E-mail: [email protected], [email protected]. Alessia Coletti and Federica Sabuzi contributed equally to the manuscript. Copyright  2018, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

Alessia Coletti et al / Journal of Fuel Chemistry and Technology, 2018, 46(9): 11211129

adsorption techniques are promising when coupled with hydrodesulfurization process. Among others, the oxidative route (ODS) for fuel desulfurization usually offers a green approach to obtain a drastic reduction in S compounds[1,13–15]. According to this process, sulphides are oxidized to the corresponding sulfoxide and/or sulfone derivatives that, being polar, can be easily removed from the fuel by extraction or adsorption. One of the main advantages related to oxidation reactions is that they occur in very mild conditions, so their applicability on an industrial-scale is feasible since no special equipment and/or dangerous reagents are required. Moreover, the system is often coupled with ionic liquids (ILs) as extraction solvents and/or catalysts, in order to improve the efficiency and appealing of such approach[13,15–17]. Ionic liquids are organic salts having a melting point below 100°C and they are widely used as catalysts and solvents because of their favourable features, such as good chemical and thermal stability, negligible vapour pressure and good ability to dissolve several inorganic and organic compounds. The low volatility and good thermal stability constitute the great advantages in using ILs as solvents because such properties allow to recycle and reuse them for further extractions. The use of ionic liquids instead of conventional, volatile organic solvents is a crucial feature since health, environmental, and economic drawbacks related to the application of solvents are reduced. Oxidation of aromatic S-containing compounds in fuels has been carried out in the presence of several catalysts, such as tungsten[12,18], molybdenum[19–21], iron[22,23] and [19,21,24–29] vanadium . In this context, it has been already demonstrated that vanadium salen and salophen complexes are promising catalysts in the oxidation of a model sulfide, i.e. methyl p-tolyl sulfide, with H2O2 and, at the same time, they showed poor reactivity toward the oxidation of alkenes, such as cyclooctene[25,30]. In this paper, an effective and sustainable method for DBT removal from fuel was presented. In particular, VVO complexes with tetradentate salen and salophen ligands were used for the selective oxidation of DBT in a three-phase system, where ionic liquids were used as extractive solvents and hydrogen peroxide as oxidant. The main advantage of such method was the high selectivity

Fig. 1

toward the removal of diaromatic sulfur-containing compounds without reducing the amount of alkenes component in the fuel.

1 1.1

Materials and methods Instrumentation

Gas chromatographic analyses were carried out with a Varian 3900 instrument equipped with a FID 1770 detector and a 30 m Supelco SPB-5 column (0.25 mm diameter and 0.25 m internal film). HPLC analyses were carried out with a Shimadzu LC-10ADvp instrument equipped with a UV-vis SPDM10Avp detector and Metachem Metaphor 5u ODS-2 column (Eluent CH3CN:H2O 3:2 v/v). 1H NMR spectra were recorded with Bruker Avance 300 MHz instrument. 1.2

Materials

All commercial reagents and solvents were purchased from Sigma-Aldrich at the highest degree of purity and were used without any further purification. Salen and salophen ligands, VV-salen and salophen complexes and ILs were prepared according to previously reported procedures [30–33].

2 2.1

Experimental Synthesis of the ligands

Two equivalents of the appropriate salicylaldehyde were dissolved in the minimal amount of boiling methanol. One equivalent of the proper diamine was added dropwise and the solution was left to cool to room temperature under stirring. The precipitate was recovered by filtration, washed with cold methanol and further purified by crystallization from ethanol. Ligands were characterized with 1H NMR, that was consistent with their structure and literature data[30,31]. Ligands were obtained in the following yields: salophen, [1,2-bis-(salicylideneamino)benzene] 93%; salen, [1,2-bis-(salicylideneamino)ethane] 88%; 5,5’-(t-Bu)2salen, [1,2-bis-(5-t-Bu-salicylideneamino)ethane] 91%.

Chemical structure of the adopted VVO-complexes

Alessia Coletti et al / Journal of Fuel Chemistry and Technology, 2018, 46(9): 11211129

Fig. 2

2.2

V-catalyzed oxidation of DBT with H2O2

Synthesis of VIV-complexes

Each Schiff base (0.5 mmol) was dissolved in 100 mL of boiling methanol. The equimolar amount of V(acac)3 (174 mg, 0.5 mmol) was dissolved in 10 mL of MeOH, added dropwise to the former solution and it was left under stirring overnight at room temperature. Afterwards a precipitated solid was collected and it was washed with diethyl ether and dried. Unreacted V(acac)3 was washed off with warm acetone. VIV-complexes purity was checked with HPLC analyses. Complexes were obtained in the following yields: SalophenVIVO 73%; SalenVIVO 89%, 5,5’-(t-Bu)2salenVIVO 67%. 2.3

Synthesis of VV-complexes

200 mg of each VIVO complex were dissolved in 30 mL of dichloromethane and kept under stirring. O2 was bubbled into the solution at 0°C for 5 min. 1.2 equivalents of trifluoromethanesulfonic acid were added, causing the precipitation of a solid. The mixture was left to reach room temperature and kept under stirring for 20 h. After centrifugation of the reaction mixture and decantation of the supernatant solution the complexes were obtained in the following yields: [SalophenVVO]CF3SO3 95%; V [SalenV O]CF3SO3 89%, [5,5’-(t-Bu)2salenVVO]CF3SO3 5%. 2.4 Synthesis of 1-butyl-3-methylimidazolium bromide, [BMIm]Br 48 mL of H2O and 16 mL of 1-methylimidazole (0.2 mol) were mixed under N2 at 85°C in an oil bath. Afterwards, an excess of 1-bromobutane (56 mL, 0.5 mol) was added dropwise under nitrogen atmosphere and the mixture was kept under stirring at controlled temperature for 4 d (when CuCl2 test for the presence of 1-methylimidazole was negative). Then, the reaction mixture was cooled and transferred into a separator funnel. After removing of the organic phase (i.e. the alkyl bromide excess), the aqueous phase was washed with ethyl acetate and water was removed under reduced pressure. After treatment with active carbon to remove orange impurities, a white solid was obtained with yield of 90%. 1H NMR spectrum of [BMIm]Br in (CD3)2CO was consistent with the one reported in literature[33]: δ 10.21 (s broad, 1H), δ

8.02 (t, 1H), δ 7.94 (t, 1H), δ 4.50–4.45 (t, 2H), δ 4.13 (s, 3H), δ 1.99–1.89 (m, 2H), δ 1.45–1.32 (m, 2H) and δ 0.97–0.92 (t, 3H). 2.5 Synthesis of 1-butyl-3-methylimidazolium Tetrafluoroborate([BMIm]BF4) 30 mL of an aqueous solution 1.66 mol/L of [BMIm]Br (0.05 mol) was added in a flask with an excess amount of NaBF4 (6.13 g, 0.056 mol). The system was kept under stirring for 4 d. Afterwards, the IL was extracted with 4 portions of 25 mL of dichloromethane and then the organic phase was washed with 3 portions of 20 mL of H2O. Bromide disappearance was checked with AgNO3 test. Organic phase was dried over Na2SO4, filtered and the solvent was removed under reduced pressure. After treatment with active carbon in 25 mL of acetone to remove impurities, a transparent ionic liquid was obtained with yield of 49%. 1H NMR spectrum of [BMIm]BF4 in CDCl3 (300 MHz) was consistent with the one reported in literature[33]: 8.94 (s, 1H); 7.30 (s, 1H); 7.26 (s, 1H); 4.22 (t, 2H,); 4.00 (s, 3H); 1.90 (m, 2H); 1.40 (m, 2H) and 0.99 (t, 3H). 2.6 Synthesis of 1-butyl-3-methylimidazolium Hexafluorophosphate([BMIm]PF6) 10.3 g of KPF6 (0.056 mol) was added to 30 mL of an aqueous solution of [BMIm]Br (0.05 mol). The system was kept under stirring for 24 h. The hydrophobic ionic liquid was extracted from the aqueous phase with dichloromethane and then washed again with water to remove KBr or unreacted [BMIm]Br traces. Bromide disappearance was checked with AgNO3 test. The organic phase was dried over Na2SO4 and filtered. After treatment with active carbon to remove impurities, a transparent ionic liquid was obtained with yield of 56%. 1H NMR spectrum of [BMIm]PF6 in substitute CD3COCD3 with (CD3)2CO (300 MHz) was consistent with the one reported in literature[33]: 8.82 (s, 1H); 7.66 (s, 1H); 7.60 (s, 1H); 4.31 (t, 2H,); 4.00 (s, 3H); 1.91 (m, 2H); 1.38 (m, 2H) and 0.94 (t, 3H). 2.7 Synthesis of 1-butyl-3-methylimidazolium Trifluoromethanesulfonate([BMIm]CF3SO3)

Alessia Coletti et al / Journal of Fuel Chemistry and Technology, 2018, 46(9): 11211129 Table 1

Oxidation reactions of DBT in acetonitrile with 2 equivalents of H 2O2 Catalyst

Temperature t/°C

Time t/h

Conversiona x/%

Selectivitya s/%

type

wmol/%

DBTO

DBTO2

[salenVVO]CF3SO3

10

20

44

45

55

[salenVVO]CF3SO3

5

20

62

50

50

[salenVVO]CF3SO3

1

20

8

85

15

[salenV O]CF3SO3

5

2

57

55

45

[salenVVO]CF3SO3

1

2

74

64

36

[5,5’-(t-Bu)2salenVVO]CF3SO3

1

10

76

47

53

[salophenVVO]CF3SO3

1

0.25

92

54

46

VO(acac)2

1

0.25

90

57

43

25

V

60

reaction conditions: DBT 0.16 mol/L, a: referred to the converted substrate

Table 2 Entry

Temperature t/°C

1 2

50

3

V-catalysed oxidation of DBT in acetonitrile

H2O2 (eq)

Catalyst V

Time t/h

Conversion x/%

Selectivity s/% DBTO

DBTO2

4

[salenV O]CF3SO3

7

97

37

63

4

[salophenVVO]CF3SO3

2.5

99

14

86

4

VO(acac)2

4

99

14

86

V

4

4

[salenV O]CF3SO3

2.25

96

40

60

5

4

[salophenVVO]CF3SO3

1.5

98

23

77

6

4

VO(acac)2

1

98

22

78

4

[salenVVO]CF3SO3

2

98

37

63

7a

60

a

6

[salenV O]CF3SO3

3.5

99

13

87

9a

6

[salenVVO]CF3SO3b

3

86

8

92

10a,c

6

[salenVVO]CF3SO3

4.5

86

16

84

11a

6

[salenVVO]CF3SO3

3

86

8

92

2,5

86

19

81

8

a,c

12

70

6

V

V

[salenV O]CF3SO3

reaction conditions: DBT 0.16 mol/L, catalyst 1%; a: DBT 0.05 mol/L; b: catalyst 0.5%; c: addition of cyclooctene 0.05 mol/L as competitive substrate, : referred to the converted substrate

15.6 g of LiCF3SO3 (0.1 mol) was added to 46 mL of an aqueous solution of [BMIm]Br (0.1 mol). The system was kept under stirring for 24 h. The ionic liquid was extracted several times from the aqueous phase with dichloromethane and then washed again with small portions of water to remove inorganic salts. Bromide disappearance was checked with AgNO3 test. The organic phase was dried over Na2SO4 and filtered. After treatment with active carbon to remove impurities, a transparent ionic liquid has been obtained with yield of 45%. 1H NMR spectrum of [BMIm]CF3SO3 in CDCl3 (300 MHz) was consistent with the one reported in literature[33]: δ 9.32 (s broad, 1H), δ 7.80 (t, 1H), δ 7.72 (t, 1H) δ 4.26–4.21 (t, 2H), δ 4.02 (s, 3H), δ 1.95–1.85 (m, 2H), δ 1.47–1.34 (m, 2H) and δ 1.02–0.97 (t, 3H). 2.8 Synthesis of 1-butyl-3-methylimidazolium bis(trifluromethanesulfonyl) imide ([BMIm]Tf2N)

15.3 g of [BMIm]Br (0.07 mol) have been dissolved in 30 mL of water and 20.02 g of LiNTf2 (0.07 mol) was added to this solution. The system was kept under stirring for 48 h. At the end of the reaction, two phases were formed due to the formation of the hydrophobic ionic liquid. The IL was extracted with dichloromethane and then washed again with water to remove inorganic salts traces. The organic phase was dried over Na2SO4 and filtered. The solvent was removed under reduced pressure. After treatment with active carbon for 24 h to remove impurities, a transparent ionic liquid was obtained. Yield, 90% 1H NMR in CDCl3 (300 MHz) was consistent with the one reported in literature[33]: δ 8.89 (s broad, 1H), δ 7.62 (t, 1H) δ 7.68 (t, 1H) δ 4.24–4.19 (t, 2H), δ 3.99 (s, 3H), δ 1.94–1.83 (m, 2H), δ 1.47–1.34 (m, 2H) and δ 1.02–0.97 (t, 3H). 2.9 Synthesis of 1-propyl-3-methylimidazolium bis(trifluromethanesulfonyl) imide ([PMIm]Tf2N)

Alessia Coletti et al / Journal of Fuel Chemistry and Technology, 2018, 46(9): 11211129 Table 3

V-catalysed oxidation of DBT in ionic liquids Selectivitya s/%

Ionic liquid

H2O2 (eq)

Time t/h

Conversion x/%

[BMIm]CF3SO3

5

3.5

0

0

0

[BMIm]PF6

4

2

96

2

98

[BMIm]PF6

6

12

100

0

100

[BMIm]PF6a

4

2

94

40

60

[BMIm]PF6b

4



97

9

91

DBTO

DBTO2

reaction conditions: DBT 0.16 mol/L, cat 0.5%, t=60°C, H2O2 4 eq.; aaddition of COT 0.16 mol/L; b: addition of cyclohexane 0.16 mol/L, a: referred to the converted substrate

Fig. 3

Tri-phase system implemented for desulfurization of a model fuel

[PMIm]Tf2N was prepared by anion exchange of the corresponding bromide and it was characterized by 1H NMR in CDCl3 and it was consistent with literature data[32]: δ 8.41 (s broad, 1H), δ 7.57 (t, 1H) δ 7.48 (t, 1H) δ 4.15–4.07 (t, 2H), δ 3.98 (s, 3H), δ 1.82–1.75 (m, 2H) and δ 1.05–0.98 (t, 3H). 2.10

Oxidation of dibenzothiophene in CH3CN

Dibenzothiophene was dissolved in 5 mL of acetonitrile together with the correct amount of V-catalyst. Hydrogen peroxide was then added and the solution was kept under stirring at controlled temperature until disappearing of H 2O2 monitored by starch iodide paper. 100 L of solution was diluted in a 1 mL volumetric flask and the resulting solution was then analyzed by GC and HPLC with naphthalene as external standard. 2.11

Oxidation of dibenzothiophene in ionic liquids

Oxidation reactions were carried out in a 5 mL Schlenk reactor. Dibenzothiophene was dissolved in 3 mL of ionic liquid together with the V-catalyst and hydrogen peroxide. The solution was kept under stirring at controlled temperature until the complete consumption of H2O2. 100 L of solution was diluted with dichloromethane in a 1 mL volumetric flask and filtered over SiO2. The resulting solution was then analyzed by GC and HPLC with naphthalene as external standard. 2.12

Tri-phase system reactions

2 mL of ionic liquid was inserted in a 5 mL Schlenk reactor together with 2 mL of benzine containing DBT and COT in known concentrations. H2O2 was then added and the solution was kept under stirring at the appropriate temperature (with standard heating or MW irradiation). Quantitative analyses of reagents and products in the different phases were carried out. Benzine solution was separated and diluted with dichloromethane in a 5 mL volumetric flask with a known amount of naphthalene as external standard and analysed by GC. 100 L of the ionic liquid was dissolved in 5 mL volumetric flask with dichloromethane containing naphthalene as external standard, and the solution was filtered over SiO 2 and analyzed by GC and HPLC.

3

Results and discussion

Diaromatic sulfur containing compounds are present in high amount into fuels. For this reason, in this study dibenzothiophene (DBT) was chosen as model substrate, whilst [salophenVVO]CF3SO3 (1) and [salenVVO]CF3SO3 (2) together with its derivative [5,5’-(t-Bu)2salenVVO]CF3SO3 (3) were used as catalysts (Figure 1) due to the promising results previously obtained in the oxidation of methyl p-tolyl sulfide[25,30]. The oxidation reaction of dibenzothiophene was extensively studied in acetonitrile, changing reaction conditions such as temperature, equivalents of oxidant, nature and amount of the catalyst (Figure 2, Table 1). In order to underline the importance of the ligand, reactions were carried out with a reference vanadium(IV) complex, i.e. VO(acac)2.

Alessia Coletti et al / Journal of Fuel Chemistry and Technology, 2018, 46(9): 11211129 Table 4

Tri-phase system for desulfurization of a model fuel Selectivity s/%

Ionic liquid

Catalyst

Time t/h

Conversion x/%

[BMIm]PF6

[salenVVO]CF3SO3

24

62

30

70

[BMIm]PF6

[salophenVVO]CF3SO3

24

98



100

[BMIm]BF4

[salophenVVO]CF3SO3

5

a





16

98

45

55

[BMIm]Tf2N

V

[salophenV O]CF3SO3

DBTO

DBTO2

reaction conditions: DBT 0.16 mol/L, COT 0.16 mol/L, H2O2 4 equivalents, catalyst 0.5%, t=60°C; a: 60% of DBT extracted by IL, :.referred to the converted substrate

In this complex, vanadium was in its IV oxidation state, but in the presence of H2O2 it was immediately oxidized to V(V), so the comparison with salen and salophen based V(V) complexes was possible. Dibenzothiophene oxidation leads to the formation of the corresponding sulfoxide (DBTO) and sulfone (DBTO 2), depending on reaction conditions. Increasing the reaction temperature from 25 to 60°C results in significant decrease of reaction times and higher conversions of DBT, particularly in the presence of VO(acac)2 and [salophenVVO]CF3SO3 complexes although without significant selectivity. Interestingly, at 60°C only 1% of catalyst is required to significantly convert the substrate. To enhance the conversion of dibenzothiophene and, above all, the selectivity toward sulfone product, the reactions were carried out with larger amounts of oxidant (Table 2). The use of four equivalents of H2O2 increases the conversion of the substrate although the reaction is never completely selective towards DBTO2 (entries 1–7), whilst the addition of two more equivalents of hydrogen peroxide increases the reaction time since more oxidant have to be consumed (entries 8–12). In order to test the effect of the substrate concentration, the DBT amount is changed from 0.16 to 0.05 mol/L (entries 7–12), obtaining no significant modification in yield and selectivity. The increase of the temperature from 60 to 70°C causes shorter reaction times with lower substrate conversion, suggesting that in such conditions the decomposition of H2O2 competes with oxidation reaction (entries 8 and 11). The concentration of the catalyst results also important to minimize the decomposition of the oxidant. From a comparison between entries 8 and 9, it is evident that the selectivity toward sulfone increases using less catalyst likely due to the presence of more H2O2 in solution. Finally, to evaluate the selectivity of the reaction, cyclooctene (COT) is added as competitive substrate in equimolar amounts. In these conditions, less than 3% of the corresponding epoxide is detected, thus confirming the selectivity of the reaction toward the oxidation of sulfur containing compounds (entries 10 and 12).

The most interesting features derived from the experimental data reported in Tables 1 and 2 are that [salophenVVO]CF3SO3 is more efficient than [salenVVO]CF3SO3. Moreover, its activity in terms of reaction rate is similar to that of the reference compound VO(acac)2. This result is in agreement with theoretical calculations that suggests a simil-planar conformation for salophen vanadium complexes and a bent conformation for the salen ones[30]. Such a difference in the structure of the catalysts may explain the higher reactivity of salophen complexes that are likely more easily available from the sterically hindered substrate dibenzothiophene. On the basis of the best reaction conditions found for the oxidation of dibenzothiophene in acetonitrile, i.e. those that gave higher conversion of the substrate, the reaction was performed using ionic liquids as solvents. The aim was to build up a catalytic system in which the oxidation of sulfur containing compounds and their extraction by ionic liquids were coupled with the aim to desulfurize fuels. An hydrophilic and an hydrophobic ionic liquid, [BMIm]CF3SO3 and [BMIm]PF6 respectively, were synthesized according to previously reported literature procedure[33] and used as extraction solvents. The synthesis of ILs required a two-steps sequence: quaternization and ionic exchange. In the quaternization step, between 1-methylimidazole and 1-bromobutane, the corresponding salt was obtained. Afterwards, the bromide counterion was exchanged with an inorganic salt, namely KPF6 or LiCF3SO3, to obtain the desired product. Each step required purification over active carbon to remove impurities. Results of DBT oxidation in ILs are presented in Table 3. In [BMIm]CF3SO3 no oxidation reaction is observed. In this case, the competition between the substrate and the counter-ion CF3SO –3 provided by the solvent for the complexation of the catalyst or the peroxometal complex can take place, and thus deactivating the catalyst[30]. In [BMIm]PF6 the conversion of DBT is almost quantitative using four equivalents of the oxidant and the reaction is selective toward DBTO2. The addition of two more equivalents of H2O2 increases reaction times. Such result is particularly interesting since the sulfone is better extracted by ionic liquids with respect to the sulfoxide[13].

Alessia Coletti et al / Journal of Fuel Chemistry and Technology, 2018, 46(9): 11211129 Table 5

Oxidation reaction with and without MW treatment

Ionic liquid

Temperature t/°C

MW

Time t/s

Conversion x/%

Extraction of DBT-ox /%

[BMIm]PF6

60

no

86400

98

>90

[BMIm]Tf2N

60

no

57600

98

>98

[BMIm]PF6

120

yes

860

97

>93

[BMIm]Tf2N

120

yes

780

96

>98

[PMIm]Tf2N

100

yes

820+820

98

>98

reaction conditions: DBT 0.16 mol/L, COT 0.16 mol/L, H2O2 4 equivalents, [salophenVVO]CF3SO3 0.5%, fuel:IL=1:1

Table 6

Oxidation reaction with and without MW treatment

Temperature t/°C

Heating source

Power/W

Conversion x/%

50

MW

21

49

50

std



6

70

MW

27

69

70

std



9

90

MW

32

88

90

std



15

100

MW

35

98

100

std



18

V

reaction conditions: DBT 0.16 mol/L, COT 0.16 mol/L, H2O2 4 equivalents, [salophenV O]CF3SO3 0.5%, fuel:IL=1:1

In these conditions, cyclooctene was added as competing substrate in order to test the selectivity of the oxidation reaction towards the sulfur containing substrate and consequently the stability of unsatured compounds in the catalytic system. Even though the reaction is less selective towards sulfone, a very small amount of alkene is converted into the corresponding epoxide (up to 3%). The change in selectivity is not due to the different polarity that the addition of cyclooctene confers to the reaction medium. In fact, when the same amount of cyclohexane was added in place of cyclooctene, sulfone is formed again as the major product. Being possible to perform the reaction in hydrophobic ionic liquids, a tri-phase oxidative extraction process was implemented for the desulfurization of a model fuel, where DBT and cyclooctene were added in benzine, as illustrated in Figure 3. The aim was to combine sulfur oxidation in the fuel and extraction of the oxidized products in a single process. In this experiment, the catalyst was dissolved in the ionic liquid phase and the same volume of the model fuel was added. A very small amount of hydrogen peroxide was finally introduced in the system that resided between the ionic liquid and the organic phase, thus leading to the formation of the three-phases system. Vigorous stirring of the mixture allowed the phases to be in contact. The obtained results are collected in Table 4. Also in the tri-phase system, [salophenVVO]CF3SO3 is more active than [salenVVO]CF3SO3. Even adding only 4 equivalents of hydrogen peroxide, DBT is selectively converted into the corresponding sulfone, which is completely

removed from the organic phase by extraction in ionic liquids. Cyclooctene is recovered in the benzine and only small traces of the epoxide are detected by GC. When the hydrophobic ionic liquid [BMIm]BF4 is tested as solvent in this system, no oxidation of organic substrate is detected and only 60% of DBT is extracted by the ionic liquid, confirming the importance of coupling the oxidation with the extraction processes. At this point, a negative note has to be addressed with respect to the described protocol. It is clear that [BMIm]PF6 allows to obtain the most active desulfurization system, and unluckily, at the end of the reaction the media are strongly acid likely because formation of HF from decomposition of the ionic liquid. As a consequence, it is possible that DBT is partly decomposed in acid conditions since no complete recovery of the initial moles of DBT and its derivatives is achieved. To further improve the efficiency of the system, microwave (MW) irradiation was adopted as energy source instead of conventional heating. In fact, it was already demonstrated that the coupling between MW and ILs often provided significant increase in yield and selectivity of organic processes [34,35]. Experiments were carried out using [BMIm]PF6, [BMIm]Tf2N and [PMIm]Tf2N as ILs, while [salophenVVO]CF3SO3 was chosen as catalyst because of its higher catalytic activity with respect to the salen ligand in the previously described system. In these experiments the catalyst was dissolved in the ionic liquid, and in the same reaction vial, the model benzine (in which COT and DBT were added in known concentrations) was then added. After H2O2 addition, the reaction mixture was

Alessia Coletti et al / Journal of Fuel Chemistry and Technology, 2018, 46(9): 11211129

placed in the microwave reactor and heated at 100 or 120°C. The petrol phase thus obtained was investigated by GC and the results are reported in Table 5. Results show that microwave activation does not affect substrate conversion but it considerably decreases reaction time. Such feature makes the process even more beneficial and innovative with respect to the standard heating. To get a direct comparison between standard heating and MW application, a series of experiments was carried out in [PMIm]Tf2N. The reaction mixtures were heated by immersion in a thermostatic bath at selected temperature in the range 50–100°C (standard heating) or they were submitted to MW with a power from 21 to 35 W (average) at the same temperatures for a time of 1000 sec. The results are reported in Table 6. The results unambiguously indicate that MW irradiation considerably increases reaction rates and conversions in comparison with a standard heating at the same temperature. Among the other, MW application at 35 W and 100°C leads to the almost quantitative DBT conversion in just 1000 sec. These results show that the proposed protocol is a valid, cheap and fast alternative to hydrodesulfurization methods, and able to lead to a drastic reduction of S-content in fuels without affecting the alkenes amount.

4

Conclusions

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Oxidative fuel desulfurization was successfully achieved using a cheap and sustainable process. In particular, VVO-salophen complex resulted highly active toward DBT oxidation in the presence of an excess of H2O2, leading to the almost quantitative substrate conversion and showing high selectivity toward the formation of the corresponding sulfone. The use of ionic liquids allowed to combine in the same process both the oxidation of the substrate and the extraction of the corresponding product, i.e. the sulfone, and thus significantly lowering the amount of sulfur content in the fuel. Finally, MW irradiation resulted a powerful tool to achieve nearly complete DBT oxidation in a model fuel in just 1000 seconds. Importantly, the reaction was not effective toward olefins, and hence, the proposed system was considered an easy, rapid, clean and selective process to achieve fuel desulfurization without changing the alkenes content in the fuel, which was fundamental for its high quality.

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