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Study on the electrochemical oxidation desulfurization behavior of model diesel on AAO-CeO2 nanotubes Xiaoqing Du, Jiao Liu, Hong Chen, and Zhao Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03629 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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Study on the electrochemical oxidation desulfurization behavior of model diesel on AAO-CeO2 nanotubes Xiaoqing Dua, Jiao Liub, Hong Chenc, Zhao Zhang a,* a

Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang, 310027, China

b

c

College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, 410082, China

School of Materials Science and Energy Engineering, Foshan University, Foshan, Guangdong, 528000, China

Abstract: AAO-CeO2 nanotubes have been used as anode catalyst to remove sulfur from the model diesel composed of 1000 ppm BT, 600 ppm DBT and 300 ppm DMDBT. During the desulfurization process, BT can be removed by two main routes, ie., be oxidized to sulfone (BTO2) and to sulfates (SO2− 4 ), whilst for DBT and DMDBT the main removal way is only to be oxidized to sulfates. The desulfurization efficiency of 1000 ppm BT, 600 ppm DBT and 300 ppm DMDBT as individuals are 98.07%, 96.82% and 92.65% which are much higher than their respective desulfurization efficiency in the model diesel (mixture of BT, DBT and DMDBT). Density functional theory (DFT) calculations and cyclic voltammetry (CV) are further used to study the electrochemical oxidation desulfurization behavior of the three sulfides respectively, the results show that there may exist a critical point for sulfur concentration, when the concentration is lower than the point, the desulfurization efficiency increases with increasing the sulfur concentration, whereas, if higher than the point, the desulfurization efficiency shows an opposite trend.

Keywords: model diesel; electro-oxidation desulfurization; cyclic voltammetry; critical concentration _________________________________________________

* Corresponding author. Tel: 86-13805751827, E-mail: [email protected]

1

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1 Introduction Organic sulfides in diesel fuel pose a great threat to the environment and public health because the combustion of these compounds will directly lead to acid rain and hazy weather [1]. Meanwhile, the amount of sulfur compounds in exhaust gases from the diesel engines is far from meeting the emission standard, especially in the recent years, the regulation has become much more stringent [2,3]. In order to improve the environment and also to satisfy the rigorous standard, more innovate technologies are needed to produce cleaner diesel fuels. During the past few years, many desulfurization methods have been adopted, such as hydro-desulfurization (HDS) [4-8], adsorption-desulfurization [9-11], extraction-desulfurization [12-15] and bio-desulfurization [16], among which HDS is most widely used both in laboratory and in factory. However, HDS process needs high temperature, high pressure, high operation costs and copious amounts of hydrogen [17]. Furthermore, this process is not able to remove the heterocyclic sulfur compounds such as benzothiophene (BT) and its derivative effectively [18]. As one of the alternative desulfurization technologies, electrochemical oxidative desulfurization (EOD) has drawn much attention recently. Unlike the HDS and normal oxidative desulfurization which needs a large amount of oxidant consumption and storage[19-23], and is always accompanied by a great deal of wastewater discharge [19, 24], EOD has the ability not only to reduce the sulfur content of diesel fuel at normal temperature and pressure [20, 25-27] without consuming oxidants [23-28], but also to control the products of desulfurization to a certain extent by applying different potential [29] or to control the desulfurization efficiency via applying appropriate current [30]. Up to now, EOD has been investigated by many researchers. Wang et al. [18, 29] proposed to use β-PbO2/C or CeO2/C as anode to oxidize organic sulfides (ethanethiol, ethyl thioether and thiophene) in gasoline to a soluble sulfur compound by applying the appropriate voltage, and the total desulfurization efficiency reaches 83.9%. Tang and his coworkers [24, 31-33] reported that, when using electrochemical oxidation and extraction technologies to remove the sulfides 1-heptanethiol [24, 31-32] and methyl tert-butyl ether (MTBE) [33] in kerosene, the desulfurization efficiency is higher than 90%. Literature [24] also 2


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proposed that the main sulfide in kerosene 1-heptylmercaptan can be oxidized to 1-heptanesulfonyl chloride by using the electrolyte sodium chloride (NaCl) aqueous solution as an indirect oxidant. Nowadays, most researches on EOD are focused on analyzing the influences of electrode materials [18, 24, 29, 34] or the external factors (such as temperature, reaction time or electrolytic voltage) on the desulfurization efficiency [18, 24, 35], whereas the influence of the sulfide structure itself (such as bond strength of C-S in sulfides) or the desulfurization modes (desulfurize several sulfides simultaneously or separately) on the desulfurization efficiency has been little investigated. However, both the characteristic structure of sulfide and the steric hindrance [36-38] between different sulfides will markedly influence the desulfurization efficiency. Additionally, most researches on oxidation desulfurization of DBT proposed that the final oxidation product of DBT is DBTO2 [22, 34, 36-37, 39-40] rather than 2− soluble sulfides SO2− 4 , and only few investigators proposed that the adsorbed thiophene can be oxidized to SO4 and

CO2 in acetonitrile and n-Bu4NBF4 at platinum electrodes [41-42]. Meanwhile, our previous study [35] found that, when using AAO-CeO2 composite nanotube array as anode, the main oxidation product of model diesel is SO2− 4 . Therefore in order to confirm the feasibility of the desulfurization method and the accuracy of the results we proposed before [35] and also find out the main desulfurization routes or products of BT, DBT and DMDBT, respectively, electrochemical oxidation desulfurization of model diesel by using AAO-CeO2 nanotube array as anode has been carried out. Density functional (DFT) theory calculation and cyclic voltammograms (CV) are further used to study the individual electro-oxidation desulfurization behavior of the three sulfides, the influences of C-S bond strength and the concentration of the three sulfides on desulfurization efficiency are also analyzed, based on which a conclusion about the critical point of sulfur concentration is proposed.

2 Experimental sections 2.1 Chemical materials. The adopted model diesel was prepared by dissolving several sulfur-containing heterocyclic compounds in suitable 3


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solvent (Table 1), which is almost consistent with the real diesel from Petro China Hangzhou Co. Ltd [35]. Ce(NO3)3·6H2O (AR, 99.5%), NH4NO3 (AR, 99.5%), cyclohexane (AR, 99.5%), benzothiophene (BT, GC, 99.9%), BT sulfone, (BTO2, GC, 99.9%) dibenzothiophene (DBT, GC, 99.9%), DBT sulfone (DBTO2, GC, 99.9%), 4,6-dimethyldibenzothiophene (DMDBT, GC, 99.9%) and DMDBT sulfone (DMDBTO2, GC, 99.9%) were bought from Shanghai Aladdin Reagent Co., Ltd. All reagents were used without further purification. The used solvent was the deionized water for the electrochemical oxidation supporting electrolyte, whereas cyclohexane for the model diesel and the gas chromatography detection. Table 1 Main properties of Model Diesel Content of main compositions, ppm Density (20 C), kg/m3

BT

DBT

4,6-DMDBT

S content，ppm

835.0

1000

600

300

412.3

2.2 Experimental methods. The electrochemical oxidation desulfurization experiments of diesel were carried out in a home-made electrolytic cell shown in Figure 1. Ce(NO3)3 aqueous solution (0.1 M) was used as supporting electrolyte, a large Pt foil was used as counter electrode and the working electrode was made up of anodic alumina oxides and ceria nanotubes arrays (AAO-CeO2 NTs) as used in our previous study [35]. In the electrochemical oxidation process, 20 ml model diesel was mixed with 80 ml the above Ce(NO3)3 solution by magnetic stirring. A constant voltage (3.5 V) was applied to the working electrode, after electrochemical oxidation at 35 °C for 120 minutes, the mixed solution was separated by a separating funnel, and the obtained organic sulfides were washed by N, N–Dimethylformamide (DMF) for 3 times, the model diesel and the supporting electrolyte Ce(NO3)3 solution after desulfurization were collected for further analysis.

4


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Figure 1 Experimental setup for electrochemical oxidation desulfurization For CV experiment, a conventional three-electrode cell was used. The working electrode was AAO-CeO2 NTs (0.6 cm2), Pt foil and a saturated calomel electrode (SCE) separated from the solution by salt bridge were used as auxiliary electrode and reference electrode, respectively. The electrolyte was a mixture of 80 ml 0.1 M Ce(NO3)3 aqueous solution and 20 ml different sulfides (BT, DBT or DMDBT) cyclohexane solution under continuous magnetic stirring. The scan started from -1.0 V to 2.7 V (vs. SCE) with scan rate being 0.1 Vs-1 at 35 °C. Prior to CV records, working electrodes were first immersed in the electrolyte for a period of time to obtain a stable open circuit potential (Eocp). 2.3 Analysis Methods. The sulfur compounds before and after desulfurization were analyzed by Shimadzu gas chromatography equipped with a flame ionization detector (GC-FID, GC 2014, Japan) and a capillary column SH-RTx-5. The specific process began with the column temperature starting at 120C for 3 min, then heated at a rate of 3C per min to 275C and lastly stayed at 275C for 3 min. The concentrations of BT, DBT and DMDBT in oil were identified with the corresponding standard samples. The properties of model diesel before and after desulfurization was also analyzed by gas chromatography-mass spectrometry (GC-MS, 7890 B GC system with a HP-5MS UI capillary column (30 m x 0.32 mm x 0.25μm) and Agilent 5977 A MSD) and Fourier transform infrared (FTIR) spectroscopy (iS50, Nicolet). At last the supporting electrolyte were analyzed by ion chromatography (883 Basic IC plus, Metrohm China, Ltd.), equipped with a Metrosep A Supp 5-150 anion chromatography column (150 mm×4.0 mm inner diameter, 6.1006.520 4) and the phase 5


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change of catalyst on working electrode was analyzed by XRD. The desulfurization efficiency (XS) of diesel was calculated as the following equation, XS =

S0 −ST S0

× 100%

(1)

where S0and ST were the sulfur content (μg/g) of sulfides before and after desulfurization, respectively. 2.4 DFT calculations All the DFT calculations were performed using the Cambridge Sequential Total Energy Package (CASTEP) [43-45]. Projector-augmented-wave (PAW) potentials were used to consider the electron-ion interactions, whereas the electron exchange-correlation interactions were treated using the Perdew−Burke−Ernzerhof (PBE) exchange correlation functional [46] in the scheme of generalized gradient approximation (GGA) [46]. For the calculated molecules (see Figure 2(a)-(f), (a) denote the BT molecule; (b) denote the BT molecule with a sulfur vacancy; (c) denote the DBT molecule; and (d) denote the DBT molecule with a sulfur vacancy; (e) denote the DMDBT molecule; and (f) denote the DMDBT molecule with a sulfur vacancy), a vacuum space of 20 Å placed between adjacent layers to avoid periodic interactions. K-point samplings of 1×1×1 were used for the structure relaxation and static self-consistent calculation. All of the atomic positions and lattice vectors were fully optimized using a conjugate gradient algorithm to obtain the unstrained configuration. Atomic relaxation performed until the change in total energy was less than 0.01 m eV, and all the forces on each atom were smaller than 0.01 eV/Å.

(a)

(c)

(b)

(e)

(d)

(f)

Figure 2 The full optimized molecule structure (a) denote the BT molecule; and (b) denote the BT molecule with a sulfur 6


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vacancy; (c) denote the DBT molecule; and (d) denote the DBT molecule with a sulfur vacancy; (e) denote the DMDBT molecule; and (f) denote the DMDBT molecule with a sulfur vacancy, the yellow balls in (a), (c), (e) are sulfur atoms, the black balls are carbon atoms and the grey balls are hydrogen atoms.

3. Results 3.1 Electrochemical oxidation desulfurization of model diesel The investigated model diesel is composed of 1000 ppm BT, 600 ppm DBT and 300 ppm DMDBT based on the main compositions of typical commercial diesel (Table 1), and cyclohexane is used as solvent. The group compositions of model diesel before and after desulfurization are tested using GC (gas chromatogram) technique and shown in Figure 3 (after eliminated the solvent peak), which undoubtedly indicates that, after electrochemical oxidation, the peak areas of BT, DBT and DMDBT decrease, and more peaks are presented. Whilst, the desulfurization efficiencies for BT, DBT, DMDBT and their mixture are 90.37%, 82.14%, 73.40% and 83.36% respectively, which are calculated according to the change of the corresponding GC peak area.

DMDBT

BTBTO

2

DBT

Signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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DBT

BT

After desulfurization

DMDBT

Before desulfurization 0

10

20

30

40

50

60

Retention time (min)

Figure 3 GC chromatograms of model diesel before and after desulfurization The newly appeared peaks in GC patterns after desulfurization (Figure 3) should be new compounds generated 7


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during electrochemical oxidation. Because most sulfones are chemically synthesized by the oxidation of sulfides [19, 47], the standard sample of BTO2, DBTO2 and DMDBTO2 are used to identify the new appeared peaks. It is found that the relatively large peak at the retention time of 8.15 min is BTO2 (Figure 3), which means that BT is partly electrochemically oxidized to BTO2. Whilst the peaks of DBTO2 and DMDBTO2 do not appear, which indicates that DBT and DMDBT cannot be transformed to their corresponding sulfones under the present experimental conditions.

(a) Before desulfurization

(b) After desulfurization

Figure 4 Ion chromatography of the electrolyte in model diesel before and after desulfurization In order to find out whether the sulfurs in model diesel, especially DBT and DMDBT are oxidized to other sulfur compounds, IC of electrolyte (Figure 4) and XRD of working electrode (Figure 5) before and after desulfurization are tested. It can be seen from Figure 4 that, after electrochemical oxidation desulfurization, there is a small amount of sulfate (15.02 min) in electrolyte. Since the model diesel is composed of pure BT, DBT and DMDBT, and the supporting 8


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electrolyte is only 0.1 M Ce(NO3)3 aqueous solution, the SO42- can only come from the oxidation of model diesel. Meanwhile, XRD patterns (Figure 5) show that, before desulfurization, the main phase compositions of working electrode are CeO2 and Al3Ce [35]. The peaks at 28.549°, 47.483°and 56.25 are related to CeO2 , and , the peaks at 25.281°, 41.880°and 42.539°are corresponding to Al3Ce , and . But after desulfurization, the peaks of CeO2 disappeared completely. Therefore, combining the results of IC (Figure 4) and XRD (Figure 5) techniques, it is rationally deduced that, during the electrochemical oxidation process of model diesel, CeO2 may react with sulfides in model diesel to produce amorphous sulfates (no typical XRD peaks of Ce2(SO4)3, such as peaks at 31.361°and 41.988°can be observed in Figure 5( line B)), which is another route responsible for the decreased sulfides.

b

a: CeO2

JCPDS 43-1002 29-0012 01-0208

b: Ce2(SO4)3 c: Al3Ce

Intensity / a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b

c (101) c (110) a (111)

After desulfurization (B)

c (112) c (211) a (220) a (311)

(A) Before desulfurization

20

30

40

50

60

70

80

2Deg

Figure 5 XRD patterns of working electrode in model diesel before and after desulfurization

3.2 Electrochemical oxidation desulfurization of single BT, DBT and DMDBT, respectively The desulfurization results of mixture BT, DBT and DMDBT (Figures 3-5) demonstrate that, BT is partly oxidized to BTO2, whereas the source of element S in oxidation product SO2− 4 is unclear. Therefore, in order to find out the main 9


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desulfurization products or even the transformation paths of BT, DBT and DMDBT during their electrochemical oxidation process respectively, 1000 ppm BT, 600 ppm DBT and 300 ppm DMDBT are dissolved in cyclohexane separately and oxidized under the same conditions as those used for model diesel desulfurization. Figure 6 shows the FTIR spectra of the electrolyte containing the three sulfides before and after oxidation desulfurization. After oxidation, all the similar peaks at 1694.17 cm-1 (Figure 6a), 1695.66 cm-1 (Figure 6b) and 1694.08 cm-1 (Figure 6c) are attributed to C=O in extraction solvent DMF. For BT, the peaks at 659.08 cm-1 and 1085.19 cm-1 are corresponding to the symmetrical and antisymmetric stretching vibration of sulfate radical respectively, and the peaks at 1145.23 cm-1 and 1383.94 cm-1 come from the stretching vibration of sulfone (Figure 6a). While for DBT and DMDBT, peaks at 1083.18 cm-1, 658.34 cm-1 (Figure 6b) and at 1086.10 cm-1, 658.96 cm-1 (Figure 6c) also stand for the symmetrical and antisymmetric stretching vibration of sulfate radical. The above FTIR results indicate that, during the electrochemical oxidation process, the sulfur in BT is converted to sulfone and sulfate, whilst DBT and DMDBT are partially oxidized to their corresponding sulfates.

(a)

T%

BT before desulfurization

BT after desulfurization

1000 -1

Wavenumber (cm )

10


659.08

2000

1085.19

3000

1045.23

1694.17

4000

1383.94

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

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(b)

T%

DBT before desulfurization

DBT after desulfurization

658.34 1083.18

1695.66

4000

3000

2000

1000

0

-1

Wavenumber (cm )

(c)

T%

DMDBT before desulfurization

DMDBT after desulfurization

4000

3500

3000

2500

2000

1500

1000

658.96

1086.10

1694.08

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

500

0

-1

Wavenumber (cm )

Figure 6 FTIR spectrums of 1000 ppm BT (a), 600 ppm DBT (b), 300 ppm DMDBT (c) before and after oxidation desulfurization, respectively In order to further testify the conclusions obtained by FTIR technique (Figure 6), GC-MS of the oxidation products and IC of the electrolyte after desulfurization are also conducted. Figures 7-9 show the GC-MS chromatograms of 1000 ppm BT, 600 ppm DBT and 300 ppm DMDBT before and after desulfurization, and also the molecular ion peaks obtain from different retention times. There are two main peaks in the total GC-MS chromatogram after the oxidation desulfurization of BT (Figure 7a), the molecular ion peaks of these two main compounds appeared at m/z 134.0 (Figure 7b) and m/z 166.2 (Figure 7c), which correspond to BT and BTO2, respectively (the small peak at m/z of 82.1 and other 11


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peaks with smaller mass-to-charge ratio in Figure 7c all come from cyclohexane). While for DBT (Figure 8) and DMDBT (Figure 9), before and after oxidation, there is only one main peak which comes from DBT or DMDBT, respectively, and no other characteristic peaks about their oxidation products appeared in GC-MS chromatograms. Also based on the values of mass-to-charge ratio in Figure 8b-d and Figure 9b, the other residual smaller peaks in Figure 8a and Figure 9a are from the solvent cyclohexane. The results are consistent with the conclusions obtained by FTIR (Figure 6) that after electrochemical oxidation desulfurization, only part of BT is converted to sulfone, whilst the sulfurs in DBT and DMDBT may be oxidized to other compounds.

(a)

After desulfurization BT BTO2


Signal

BTO2

Before desulfurization

BT

6.0

6.5

7.0

7.5

8.0

8.5

9.0

Time (min)

Before desulfurization 0

5

10

15

20

25

30

35

Retention Time (min)

Scan from 7.930 to 8.930 min

(b)

Scan from 5.387 to 5.552 min 134

(c)

82.1

Relative Intensity

Relative Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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73.1 67

166.2 89.1

63 60

80

108 100

m/z

55.1 120

140

160

60

80

100

120

140

160

180

m/z

Figure 7 GC-MS chromatograms of 1000 ppm BT before and after desulfurization (a), and the molecular ion peaks obtain from different retention times (b, c). 12


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(a)

(b)


DBT

184

Relative Intensity

Signal


DBT

159.1

Before desulfurization 79.1 0

5

10

15

20

25

30

35

60

92

80

162

100

120



(c)

140

160

m/z

180

200

220

240

(d)


Relative Intensity

Relative Intensity

57.1

55 71.1

69

97 60

57.1

88.1

80

100

140 120

75.1

83.1 85 97.1 111.1 140

160

180

200

220

80

240

147 221.1

120

160

200

240

280

320

360

m/z

m/z

Figure 8 GC-MS chromatograms of 600 ppm DBT before and after desulfurization (a) and the molecular ion peaks obtain from different retention times (b-d).

DMDBT

(a)

(b)


212.1

Relative Intensity

After desulfurization Signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

DMDBT

Before desulfurization 197 92.1 0

5

10

15

20

25

30

35

60

80

105.1

100

178 152.1 166.1 120

140

m/z


13


160

180

208 200

220

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Figure 9 GC-MS chromatograms of 300 ppm DMDBT before and after desulfurization (a) and the molecular ion peaks obtain from different retention times (b). The desulfurization efficiencies (Figure 10) of single BT, DBT or DMDBT are also calculated from the GC results (Figure 7-9), which obviously shows that the desulfurization efficiency of single BT, DBT or DMDBT is much higher than that when they are coexisting (in model diesel, section 3.1). This result is just in accordance with that obtained by Mei et al [28] that the oxidation desulfurization efficiency of diesel increases with decreasing the total concentration of organic sulfides.

98.07%

Desulfurization Efficiency / Xs (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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96.82%

92.65%

BT

DBT

DMDBT

Sulfides Types

Figure 10 Desulfurization efficiencies for 1000 ppm BT, 600 ppm DBT and 300 ppm DMDBT, respectively Figure 11 shows the IC chromatography of the supporting electrolyte after oxidation desulfurization. When compared these IC chromatography (Figure 11a-c) with that of the electrolyte before desulfurization (Figure 4a), it is obvious that all of the sulfur in BT, DBT and DMDBT can be partially oxidized to SO2− 4 . Table 2 lists the contents of 2− both the IC measured SO2− 4 in electrolyte after oxidation desulfurization (E (SO4 )) and the theoretically calculated 2− 2− 2− SO2− 4 under the assumption that the sulfur in three sulfides are totally converted to SO4 (T (SO4 )). The ratio of E (SO4 ) 2− 2− to T (SO2− 4 ) is defined as the convert ratio of sulfur: E (SO4 )/T (SO4 ). It is easy to find that 93.44% sulfur in 1000 ppm

BT, 90.09% sulfur in 600 ppm DBT and 85.93% sulfur in 300 ppm DMDBT are oxidized to SO2− 4 during the 14


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electrochemical oxidation process. It should be mentioned that, although there exists small difference between the above convert ratio of sulfur (Table 2) and the desulfurization efficiency calculated from the GC results (Figure 10) for the three typical sulfides, the relative relationship between these data is doubtless. Therefore, the IC results give a further certification about the conclusions obtained by FTIR and GC-MS that the vast majority of BT, DBT and DMDBT are oxidized to SO2− 4 .

(a) 1000 ppm BT

(b) 600 ppm DBT

(c) 300 ppm DMDBT

15


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(d) 600 ppm DMDBT

(e) 1000 ppm DMDBT

Figure 11 Ion chromatography of the electrolyte in different sulfides after desulfurization (a) 1000 ppm BT, (b) 600 ppm DBT, (c) 300 ppm DMDBT, (d) 600 ppm DMDBT, (e) 1000 ppm DMDBT Table 2 The concentration of SO2− 4 in supporting electrolyte and electrode surface after desulfurization Ions (g/l)

Experimental value NO3

E(SO2 )

Sulfides

Theoretical value

E(SO2 ) /

T(SO2 )

T(SO2 )

-

Blank

18.019

None

0

0

1000 ppm BT

19.971

0.669

0.716

93.44%

600 ppm DBT

19.985

0.282

0.313

90.09%

300 ppm DMDBT

18.825

0.116

0.135

85.93%

600 ppm DMDBT

19.863

0.237

0.272

87.13%

1000 ppm DMDBT

18.992

0.402

0.453

88.74%

16


Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The IC chromatography of 600 ppm and 1000 ppm DMDBT are also tested (Figure 11d-e), and the convert ratio of sulfur in these concentrations are also calculated (Table 2). The convert ratio of sulfur in 1000 ppm BT (93.44%) is higher than that in 1000 ppm DMDBT (88.74%), which means the sulfur in BT (1000 ppm). is easier to be transformed to SO2− 4 than that in DMDBT (1000 ppm). The same result is achieved when compared the convert ratio of sulfur of 600 ppm DBT (90.09%) and 600 ppm DMDBT (87.13%). These results confirm the conclusion obtained above that sulfur in DMDBT is the hardest to be removed among these three sulfides. However, these results seem to be violating the theoretical expectation that the sulfur atom in DMDBT owns the largest electron density because of the electron-donating effect from the two methyl groups [48-49], whilst the oxidation rate constant increases with the increase of electron density [50], which theoretically infers that the sulfur in DMDBT should most easily be oxidized and DMDBT should possess the highest desulfurization efficiency. Therefore, under the hypothesis that the desulfurization efficiency is determined by the ability to convert sulfur in sulfides to other sulfur compounds, which partly depends on the intrinsic property of reactant, i.e. chemical bond's energy of C-S in the sulfides [51], DFT calculations are adopted to calculate the bond formation energy of C-S (Ebond) according to the following equations, 𝐸𝑡𝑜𝑡 = ∑𝑖 𝐸𝑖𝑎𝑡𝑜𝑚 + ∑𝑗 𝐸𝑗𝑏𝑜𝑛𝑑 𝐸𝑏𝑜𝑛𝑑 = 𝐸𝑡𝑜𝑡

∑𝑖 𝐸𝑖𝑎𝑡𝑜𝑚

(2)

∑𝑗≠𝑘 𝐸𝑗𝑏𝑜𝑛𝑑 𝑗

(3)

where, 𝐸𝑡𝑜𝑡 is the energy of an optimized molecule, 𝐸𝑖𝑎𝑡𝑜𝑚 is the energy of atom of the optimized molecule, 𝐸𝑗𝑏𝑜𝑛𝑑 is the bond energy of the optimized molecule.

17


Energy & Fuels

-279.8

-280.0

-280.079 eV

-280.054 eV

-280.2

Ebond/eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

-280.4

-280.6

-280.596eV

-280.8

-281.0

BT

DBT

DMDBT

Figure 12 The calculated relative bond formation energy of S-C bond in three molecules The DFT calculated bond formation energy of S-C in BT, DBT and DMDBT (Figure 12) undoubtedly indicates that the S-C bond formation energy in DMDBT molecule is the largest, which infers that the S-C bond in DMDBT molecule would most easily be destroyed and the desulfurization efficiency of DMDBT would be the highest if only considering the C-S bond strength of the three sulfides. Unfortunately, the DFT results are inconsistent with the experimental measured results that the desulfurization efficiency of 1000 ppm BT is much higher than that of 1000 ppm DMDBT, and also that of 600 ppm DBT is much higher than 600 ppm DMDBT (Table 2). Therefore, it is reasonable to propose that there should be other facts except the strength of C-S bond to influence the desulfurization efficiency. Moreover, the oxidation desulfurization efficiency of DMDBT increases with increasing its concentration (Table 2), which is just opposite to the result obtained by Mei et al [28] that the oxidation desulfurization efficiency of diesel increases with decreasing the total concentration of organic sulfides, and is also contrary to the result obtained by ourselves that the desulfurization efficiency of single BT, DBT or DMDBT (Figure 10) is much higher than that when they are coexisting in model diesel (section 3.1). These phenomena also undoubtedly indicate that the desulfurization efficiency is influenced by other facts except the concentration of organic sulfides. The electrode surface before and after desulfurization is also analyzed using XRD technique (Figure 13). After the 18


Page 19 of 26

oxidation of BT (Figure 13a), the original peaks at 28.549°and 47.483°, which are corresponding to crystal face (111) and (220) of CeO2, disappeared, and four new peaks at 31.361°, 36.342°, 41.988°and 68.423°appeared. Based on the results from FTIR (Figure 7) and IC (Figure 11) and our previous study [35], these new peaks come from the generated amorphous Ce2(SO4)3. The above phenomena are also observed in XRD patterns of DBT (Figure 13b) and DMDBT (Figure 13c) except that, the peaks of Ce2(SO4)3 for DBT only showed up at 36.342°and 41.988°and there is only one characteristic peak of Ce2(SO4)3 for DMDBT. Therefore, combined the obtained XRD results (Figure 13) and the GC-MS results above (Figures 7-9), it can be confirm that, during the electrochemical oxidation process of model diesel, BT can be removed by being oxidized to sulfone (BTO2) and to sulfate (SO2− 4 ), whilst the main removal way for DBT and DMDBT is only to be oxidized to sulfates.

(a)

a: CeO2 b: Ce2(SO4)3

b

c: Al3Ce Intensity / a.u

b c (110)

b b

a (111)

BT after desulfurization c (112)

a (220) BT before desulfurization

20

30

40

50

60

70

80

90

2Deg

(b)

a: CeO2 b: Ce2(SO4)3 c: Al3Ce

Intensity / a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

b

c (110)

b

a (111) DBT after desulfurization

c (112) a (220) c (211) DBT before desulfurization 20

30

40

50

60

70

2Deg

19


80

Energy & Fuels

(c)

a: CeO2 b: Ce2(SO4)3 c: Al3Ce

Intensity / a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

b

c (110) a (111)

DMDBT after desulfurization

c (211) c (112)a (220) DMDBT before desulfurization 20

30

40

50

60

70

80

2Deg

Figure 13 XRD patterns of working electrodes in (a) 1000 ppm BT, (b) 600 ppm DBT, (c) 300 ppm DMDBT before and after desulfurization

4. Discussion The oxidation desulfurization efficiency of DMDBT is the lowest (Table 2, Figure 10) among BT, DBT and DMDBT sulfides, and increases with increasing its concentration (Table 2), which are contrary to the DFT calculated results if only considering the C-S bond strength of the three sulfides (Figure 12) and also violate the results of Mei et al [28] and part of ourselves (compare Figure 10 with the desulfurization efficiency in section 3.1), respectively. In order to elucidate these discrepancies, cyclic voltammetry technique is adopted to study the oxidation process of BT, DBT and DMDBT at AAO-CeO2 electrode (Figure 14), and the corresponding parameters deduced from the CV curves are listed in Table 3. When the anode potential is lower than ca. 0.5 V or higher than ca. 2.2 V, the oxidation current at the same potential follows the order of BT (1000 ppm) > DBT (600 ppm) > DMDBT (300 ppm) (Figure 14a), which indicates that the oxidation rate and consequently the desulfurization efficiency of these three organic sulfides follows the same order (Figure 10, Table 2) since the oxidation time is settled at 120 min and the used oxidation potential is 3.5 V in our experiments. In addition, only the CV of BT shows two evident types of oxidation current (I and II with distinct turning 20


Page 21 of 26

point) (Figure 14a-b), which may be an indication of two types of oxidation reactions existing in its whole oxidation desulfurization process. As elucidated above (section 3), the oxidation products of BT consist of sulfone (BTO2) and SO2− 4 , therefore, it is rationally deduced that the two types of oxidation process should be the oxidation of BT to BTO2 and to SO2− 4 , respectively. In other words, BT can be removed by two main routes, ie., to be oxidized to BTO2 2− (region II for that the valence state of S in BTO2 is lower than that in SO2− 4 ) and to SO4 (region I) in the whole

desulfurization process.

2.5

1.0

Current / mA

(b)

a: without sulfides b: 1000ppm BT c: 600ppm DBT d: 300ppm DMDBT

b a: without sulfides b: 1000ppm BT c: 1000ppm DBT d: 1000ppm DMDBT

2.0

Current / mA

(a)

1.2

0.8

(I) 0.6

1.5

c

b 1.0

0.4

c 0.5

0.2

0.0 -1

0

1

d

d a

(II)

2

a

0.0

3

-1

0

1

2

3

Potential / V vs SCE


2.5 0.25

(c) a: without sulfides b: 600 ppm DBT c: 1000 ppm DBT

2.0

(d)

c a: without sulfiides b: 300 ppm DMDBT c: 1000 ppm DMDBT

0.20

c

1.5

Current / mA

Current / mA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1.0

0.15

b

0.10

0.05

0.5

b

a 0.00

a

0.0

-1

0

1

2

-1

3

0

1

2

3



Figure 14 Cyclic voltammetry of sulfides with different concentration in cyclohexane + 0.1 M Ce3+ at AAO-CeO2 electrodes at 0.1 V s-1 Table 3 The corresponding parameters obtain form the CV curves Types

E-ocp/ V

Eox-onset / V

21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

blank

-0.411

none

1000 ppm-BT

-0.313

0.50 for BTO2, 1.95 for SO2

600 ppm-DBT

-0.345

1.45

300 ppm-DMDBT

-0.339

1.04

1000 ppm-DBT

-0.534

1.35

1000 ppm-DMDBT

-0.593

0.94

Page 22 of 26

The onset potential at which the oxidation current starts to increase sharply (Eox-onset) is totally different for the three sulfides (Table 3). The Eox-onset can be used to evaluate whether a reaction can occur or not, and the oxidation reaction with a higher Eox-onset always means more energy is needed to trigger this reaction than that with a lower Eox-onset [52-53]. The Eox-onset of BT, DBT and DMDBT of the same concentration (1000 ppm) for SO2− 4 formation are 1.95 V, 1.35 V and 0.94 V, respectively (Table 3), which confirms the DFT calculated result that the oxidation of DMDBT to SO2− 4 should be thermodynamically the easiest among the three sulfides. However, according to the electrochemistry theory (Reaction 4), the anodic oxidation current (ja) is not only dependent on the strength of C-S bond, which mainly influences the chemical activation energy (Ea), but also depends on the reactant concentration (CR) and the electrode overpotential (), 𝑎

=

𝑛

(4)

where, n is the number of electron transferred, F is the Faraday's constant, R the ideal gas constant,

the transfer

coefficient and T the absolute temperature. Therefore, the intrinsic nature, such as the strength of C-S bond and the Eox-onset of sulfides can only determine whether the reaction can occur or not under certain experimental conditions, the reaction extent (reaction rate or current) should be mainly decided by the concentration and the steric hindrance [36-38, 54] of the reactants, and also the electrode overpotential (). Figure 14b clearly shows that at higher oxidation 22


Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

potential, DBT is the easiest to be oxidized to SO2− 4 among the three investigated organic sulfides of same concentration, whilst Figures 14c-d clearly indicates that the oxidation current increases with increasing the concentration of DBT and DMDBT. Accordingly, when adopting the real [28] or model diesel (section 3.1) of higher concentration as the desulfurization target, the interactions among the different organic sulfides should reduce with decreasing concentration, which therefore enhances the desulfurization efficiency (compare Figure 10 with the desulfurization efficiencies of BT, DBT and DMDBT in section 3.1). However, when the concentration is lower than certain critical point, the increase of sulfide concentration not only has little influence on the mutual effect or overpotential (Table 3), but markedly enhances the oxidation rate (Figures 14c-d) and therefore the convert ratio of sulfur or the desulfurization efficiency (Table 2).

5. Conclusion Electrochemical oxidation by using AAO-CeO2 NTs as catalyst can be an efficient way to remove the sulfurs in BT, DBT and DMDBT. The desulfurization efficiency of 1000 ppm BT, 600 ppm DBT and 300 ppm DMDBT as individuals are 98.07%, 96.82% and 92.65% which are much higher than their respective desulfurization efficiency in model diesel. During the oxidation desulfurization process, BT can be removed by two main routes, ie., be oxidized to sulfone (BTO2) and to sulfides (SO2− 4 ), whilst the main removal way for DBT or DMDBT is only to be oxidized to sulfates. Based on DFT and CV results, another conclusion can be deduced that there exist a critical point of sulfur concentration, when the concentration is lower than the point, the desulfurization efficiency increases with the increase of the sulfur concentration, whereas if higher than the point, the desulfurization efficiency shows an opposite trend.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Project 51771173, 21073162, 21403194 and 21363018) and Natural Science Foundation of Shandong Province (ZR2014EMQ007).

23


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Study on the electrochemical oxidation ... - ACS Publications

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