Structural Modification of Petroleum Pitch Induced by Oxidation

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Structural modification of petroleum pitch induced by oxidation treatment and its relevance to carbonization behaviors Bin Lou, Dong Liu, Yajing Duan, Xulian Hou, Yadong Zhang, Zhiheng Li, Zhaowen Wang, and Ming Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01314 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 28, 2017

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Energy & Fuels

1

Structural modification of petroleum pitch induced by oxidation

2

treatment and its relevance to carbonization behaviors

3

Bin Lou 1, Dong Liu 1, *, Yajing Duan 2, Xulian Hou 3, Yadong Zhang 1, Zhiheng Li 1, Zhaowen

4

Wang 4, Ming Li 5, *

5

1

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao,

6

Shandong 266580, People’s Republic of China

7

2

Patent Examination Cooperation Center of The Patent Office, SIPO, Henan, Zhengzhou, Henan

8

450002, People’s Republic of China

9

3

China Petroleum Engineering Company, Ltd., Beijing Company, Beijing 100085, People’s

10

Republic of China

11

4

12

Republic of China

13

5

14

Shandong 266042, People’s Republic of China

15

Corresponding Author

16

* Dong Liu, Telephone/Fax: +86-0532-86984629. E-mail: [email protected].

17

* Ming Li, Telephone: 15063083161. E-mail: [email protected]

18

Abstract: Structural modifications of petroleum pitch and its sub-fractions induced by mild

19

air-blowing modification have been monitored by elemental analysis, average molecular weight,

20

Fourier transform infrared spectrometer (FTIR), 1H nuclear magnetic resonance (1H-NMR) and

21

carbon residue. Combined with the investigation into carbonization behaviors of as-prepared

22

oxidized pitch, the relationship between oxidized modification and carbonization behaviors has

23

also been discussed. With the air blowing treatment processing, average molecular weight and

24

oxygen content grow rapidly in petroleum ether-insoluble fraction of air-blown pitch in

25

comparison with those of corresponding petroleum ether-soluble fractions, which gradually

26

enlarges distinction in carbonization reactivity and mutual solubility between this two

27

sub-fractions. As a result, although the yield of carbonized residue is dramatically promoted from

28

36.1wt% to 64.5wt% during the direct thermal carbonization process, the air blowing treatment

29

produces adverse effect on mesophase development of oxidized pitches, leading to optical texture

30

index (OTI) significantly decreasing from 35.9 to 0.6. While adding the hydrogen-donor aromatic

31

oil (HAO) into air-blown pitches is able to effectively improve the mutual compatibility among

Dongying Municipal Environmental Protection Bureau, Dongying, Shandong 257091, People’s

College of Chemical Engineering, Qingdao University of Science & Technology, Qingdao,

1

ACS Paragon Plus Environment

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the solvent sub-fractions via H-transfer reactions and dilution effect, consequently contributing to

33

the mesophase development during co-carbonization process accompanied by increasing OTI

34

value of mesophase residues to about 65 except 33.6 of PP25-HAO mixture; simultaneously, the

35

co-carbonization process still maintains the relatively high carbonized residues yield between

36

42.1wt% and 61.8wt%. Hence, a possible combined process including air blowing modification

37

and subsequent co-carbonization process can obtain mesophase products with high residue yield

38

and excellent anisotropic texture by controlling oxidized degree and selecting compatible

39

co-carbonization additives.

40

Keywords: air blowing; co-carbonization; mesophase; sub-fractions; mutual compatibility

41

1. Introduction

42

Commercial petroleum pitches and coal-tar pitches have been extensively used to

43

manufacture various carbon materials. Generally, commercial pitches can meet the requirements

44

of traditional application. However, because of their low carbon yield and poor graphitization

45

ability, the properties of commercial pitches are not suitable for more special and modern

46

applications [1-4], such as carbon fibers, needle coke or C/C composites. In order to satisfy the

47

requirement as precursor of advanced carbon material, the modified treatments are usually utilized

48

and the objective of pitch modification are generally defined as follows: (1) maintaining or

49

promoting the mesophase development; (2) improving fluidity when the mesophase products

50

fused; (3) increasing carbon residues. As compared to coal tar pitches, petroleum pitches are more

51

aliphatic, whose molecules are featured with aliphatic and aromatic constituents forming a cobweb

52

structure. Owing to such molecular structure, petroleum pitches possess higher softening points

53

with simultaneously lower carbon residues than highly aromatic coal tar pitches.

54

In the last decades, comparing with thermal process [5], adding elemental sulfur [6] or

55

oxygen blowing [7, 8], air blowing treatment featured with low cost and simple procedure [9-12]

56

is most commonly used to effectively promote the carbon residue of pitch. Machnikowskia et al

57

[15,16] reported that the influence of air-blowing treatment on the optical texture in the

58

subsequent carbonization reactions relies on the properties of starting substances and oxidized

59

conditions. But in most cases air-blown treatment deteriorates the mesophase development of

60

modified pitch [17]. That is to say, in order to develop a suitable ‘‘on demand’’ modification of

61

molecular compositions during the oxidized process, it is necessary to select both suitable 2


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feedstock and appropriate oxidized process. Normally, the feedstock is far from achieving this a

63

series of requirements for molecular composition. So the structural modification induced only by

64

air/oxygen blowing process is very difficult to achieve the improvement of mesophase

65

development and promotion of carbon residues.

66

In order to improve the mesophase development during the carbonization and fused fluidity

67

of the resultant mesophase pitch, hydrogenation modification [18,19], catalytic polymerization

68

modification [20,21] and co-carbonization modification [22,23] were also developed. Mochida [24]

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has reported a representative process of hydrogenation modification: naphthenic groups are

70

introduced into the aromatic molecules of coal tar pitch with the aid of a hydrogen donor solvent

71

like tetrahydroquinoline or catalyst, respectively; and then some of the introduced naphthenic

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groups survive in the resultant mesophase pitch. But The high cost of aromatic hydrogenation

73

limits its wide application. As far as I know, the aim of almost all hydrogenation treatment focuses

74

on improving the mesophase development. The published reports concerning increasing the pitch

75

yield through hydrogenation are rare. Co-carbonzation modification is regarded as a practical and

76

effective method, which is performed by mixing two kinds of feedstock to improve carbonation

77

properties of each feedstock. With respect to co-carbonization mechanism, the concept of

78

“Eutectic Effect” reported by Marsh and Mochida [25, 26] is commonly accepted, specially

79

including additive as seed crystal to contribute to rapid mesophase formation, regulating the

80

carbonization rate and mesogen configuration by alkyl and/or hydrogen transfer reactions during

81

the co-carbonization process as well as excellent dissolve ability of additive to decrease the

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viscosity of reaction intermediate. So the selection of matched additive is crucial according to the

83

defects in the carbonization behaviors of the other feedstock.

84

At present, it is gradually accepted that only single modified method is difficult to achieve

85

above three objectives. To produce pitch with excellent carbonization properties, several kinds of

86

modification treatments should be successively applied to feedstocks [27]. But as we known, few

87

attempts appear to have been made to achieve the compatible combination of different

88

modification treatment.

89

In this paper, several oxidized pitch with different softening point were prepared from a

90

commercial petroleum pitch by air blowing. The properties of as-treated pitches and their solvent

91

fractions as well as mesophase development during both direct carbonization and co-carbonization 3


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process were investigated to further understand the relationship between the oxidized treatment

93

and their carbonization behaviors, aiming to disclose a combined process that can promote both

94

carbon yield and mesophase development.

95

2. Experimental Section

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2.1 Feedstock. A commercial petroleum pitch as the parent pitch was obtained from Shengli

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Petrochemical Plant of Chinese Petroleum Group. The modified pitches derived through air

98

blowing treatment were used as feedstock in the subsequent carbonization process. And the

99

hydrogen-donor aromatic oil (HAO) from naphthenic base crude oil was selected as the additive

100

of co-carbonization. The properties of HAO are listed in Table 1. The relatively high C/H atomic

101

ratio 0.74 of HAO indicates that it has relative high aromaticity that contributes to the mutual

102

solubility with the colloidal system of the pitch-HAO mixtures [28]. Likewise, the SARA analysis

103

of the HAO shows that the predominant fraction was the aromatics (62.94wt%) and the saturates

104

as well as resins are only 22.26wt% and 13.69wt%, also suggesting that adding HAO rarely

105

destroys the colloidal stability of the pitches but disperses their highly reactive species. Besides,

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according to the structural parameters calculated by n-d-M method, it is seen that the percent of

107

aromatic carbon (CA) of HAO is as high as 40.4wt%, accompanied by 34.6wt% of naphthenic

108

carbon (CN) and only 25.0wt% of alkyl carbon (CP), implying that constitute molecules of HAO

109

contains not only high aromaticity but also abundant naphthenic structure attached to aromatic

110

ring.

111

Table 1 The properties of co-carbonization additive HAO Items

HAO ρ/g·cm-3

0.9944

nD20

1.5608

Mw

383.2

C/wt%

87.93

H/wt%

9.93

S/wt%

0.55

N/wt%

0.58

O/wt%

1.00

Elementary analysis

4


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n-d-M method

C/H

0.74

CA/wt%

40.4

CN/wt%

34.6

CP/wt%

25.0

Saturates/wt%

22.26

Aromatics/wt%

62.94

Resins/wt%

13.70

Asphaltenes/wt%

0

SARA analysis

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In this paper, the pitches were also separated into two kinds of solvent sub-fractions including

113

petroleum ether-soluble (PES) and petroleum ether-insoluble (PEI) fractions by extraction of

114

petroleum ether with a boiling range from 90℃ to 120℃, according to ASTM D6560-2000.

115

2.2 Air blowing treatment. According to the optimal results of air blowing treatment (seen

116

in Table S1and S2 in the Supporting Information ), the petroleum pitch was treated at temperature

117

of 280°C under an air flow of 160 L•kg-1•h-1 and constant 200 /min of stirring speed in a 1L flask

118

(seen in Figure 1). And the PP14 pitch, PP20 pitch and PP25 pitch were prepared by air blowing

119

treatment for 14h, 20h and 25 h, respectively.

120

Before air blowing treatment, the weight of empty 1L three-necks flask and the gross

121

weight of flask and parent pitch are obtained by using AGF-4000 elctronic

122

balance (made in A & D company, Japan). Likewise, when blowing time reaches, the gross weight

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of flask and oxidized pitch is also weighted. Therefore, the oxidized pitch

124

yield in Table 3 is calculated as followed:

125 126 127 128

Oxidized Pitch Yield / wt% =

!" # $% &

#'"(

# $% &

×100%

Loss of Feed / wt% = 100% - oxidized pitch yield

(1) (2)

From above formula (1) and (2), the amount of oxygen atoms involving in generation of the constituent molecules of oxidized pitch is not deducted.

5


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129 130

Figure 1. The experimental autoclave for oxidized cross-linking of the pitch

131

2.3 Carbonization process. The carbonizing processes were carried out using a 300ml

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autoclave apparatus (seen in Figure 2 ). The detailed description of operating procedure,

133

experimental design and repeatability of carbonization experiments can be found in Section 2 and

134

Section 3 of Supporting Information. In order to discuss the structural modification and its

135

relevance to subsequent carbonization, simple variable method was adopted to select the optimal

136

conditions including reaction temperature, reaction pressure, soaking time and amount of HAO

137

addition in terms of mesophase content in carbonaceous mesophase products. The corresponding

138

results are discussed in following Section 3.2 and 3.3.

139 140

Figure 2. The equipment used for carbonization

141

2.4 Characterization. Softening points (SP) used to characterize the oxidized degree of

142

derived pitch were determined using ring and ball method according to ASTM D3461 standards.

143

In order to monitoring the variations in structural compositions during the air blowing, the content

144

of elements including carbon, hydrogen, sulfur and nitrogen of samples was directly determined

6


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by the Varil EL-3 element analyzer; while the oxygen content was obtained by difference; and 1H

146

nuclear magnetic resonance (1H-NMR) spectra which were recorded on a BrukerAvance DMX500

147

spectrometer, using deuterated chloroform as solvent and tetramethylsilane (TMS) as internal

148

standard as well as the Fourier transform infrared (FTIR) spectra with an average of 32 scans and

149

a resolution of 2 cm-1 in the region of 4000-500 cm-1 which were determined on a Nicolet

150

Magna-750 FTIR spectrometer were also used. Besides, the cross-linking extent in the pitches was

151

estimated from pitch iodine up-take [29] and carbon residues of samples are determined according

152

to ASTM Method D189-05. The average molecule weight (Mw) of the solvent fraction of the

153

pitch was determined by KNAUER K-7000 vapor pressure osmometry (VPO) to distinguish the

154

varied amplitude of sub-fractions of oxidized pitch.

155

The content and shape of mesophase structure as important indicators to evaluate the quality

156

of carbonized solid residues were characterized as follow: the mesophase products were mounted

157

in epoxy resin, polished, and examined by XP-4030 polarizing microscope (Shanghai milite

158

Precise Instrument Co. Ltd, China). To a considerable level, the properties of mesophase products

159

are decided by their crystalline structure which is mainly reflected by those anisotropic optical

160

texture. So the optical texture index (OTI) used to evaluate the mesophase development was

161

calculated according to the following formula: OTI=Σfi * OTIi, where fi is the proportion of

162

different anisotropic structures and OTIi, representing the special anisotropic structure index

163

suggested by Ester [30,31], is summarized in Table 2.

164

165

Table 2. The classification of mesophase microstructures and its OTI value Microstructural Types

Feature Size/µm

OTI Value

Mosaic

＜10

1

Small domain

10～60

5

Domain

＞60

50

Large domain

length＞60，width＞10

100

3. Results and Discussion

166

3.1 Structural modification of oxidized pitches

167

3.1.1 Properties analysis of oxidized pitches

168

The variations in softening points, yields of oxidized residues, elemental compositions, 7


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169

carbon residues and iodine values of parent and treated pitches are summarized in Table 3.

170

Comparing the properties from parent pitch to PP25 pitch, the oxidized treatment results into

171

vaporization loss of less than 10% for the parent pitch but significant growth of softening point to

172

158℃, accompanied by gradually reducing iodine value from 3.984 to 3.043 nmol·g-1. Besides, it

173

is observed that the carbon residue of derived pitch dramatically increases from 21.6wt% to

174

54.6wt%. These findings clearly indicate that the air blowing regarded as effective process to

175

promote the carbon residue of pitch is mainly due to the reactions such as aromatization and

176

cross-linking among the components [32], and not just to the removal of volatile compounds.

177

Furthermore, as the oxidized reactions proceeds, the hydrogen content of the derived pitches

178

shows a progressive decrease but increase in C/H atom ratio; meanwhile the oxygen content

179

slightly increases from 0.69wt% for parent pitch to 1.81wt% for PP25 pitch. These results all

180

suggest that air blowing can induce the dehydrogenative polymerization of pitch constituents and

181

most oxygen is not incorporated into the aromatic molecules, but being eliminated as water [33,

182

34]. To obtain better insight into structural changes during the oxidation treatment, the FTIR and

183

1

H-NMR analysis were also conducted.

184

Table 3. Variations in basic properties of oxidized pitches Items

Parent pitch

PP14 pitch

PP20 pitch

PP25 pitch

Softening point/℃

50

113

136

158

Oxidized pitch yield/wt%

--

93.7

91.8

90.5

C/wt%

86.96

86.99

87.01

86.67

H/wt%

10.80

10.32

9.94

9.72

S/wt%

0.68

0.46

0.59

0.76

N/wt%

0.87

1.03

1.00

1.04

O/wt%

0.69

1.20

1.46

1.81

C/H

0.67

0.70

0.73

0.74

Carbon residue/wt%

21.6

43.5

48.4

54.6

Iodine value/mmol·g-1

3.984

3.698

3.231

3.043

Elemental analysis

185

3.1.2 FTIR and 1H-NMR characterization of oxidized pitches

186

The FTIR spectra and 1H-NMR analysis of the original pitch and modified pitches are

8


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presented in Figure 3 and Table 4, respectively. There are some distinct differences among these

188

characterization results, which contributes to obtain detailed information about the oxidized

189

mechanism at different periods of air blowing treatment.

190

According to Figure 3, with the softening point of the pitch increasing, one significant

191

difference is that the absorption peaks of the aromatic C=C stretching near 1600cm-1 and the

192

aromatic C-H out-of-plane bending stretching between the region of 700-900cm-1 becomes more

193

and more evident, which indicates that reactions of dealkylation, cycli-aromatization and

194

condensation of aromatic rings are induced during the air blowing treatment, contributing to the

195

growth of softening point of the derived pitch. Moreover, new two absorption bands near

196

3350cm-1 attributed to O-H stretching and 1693cm-1 assigned to C=O stretching are seen in spectra

197

of both PP20 pitch and PP25 pitch. And, another new band at 1108.9 cm-1, which is assigned to

198

phenoxy or ether stretching [34], is significantly intensified only in PP25 pattern. It shows that the

199

rise of softening point in PP20 and PP25 pitch, to a larger extent, is attributed to the cross-linking

200

of pitch components by introducing the oxygen functional groups.

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Energy & Fuels

(a)

1604.4

1375.0

1456.0

2856.1

2923.6

(b) 1604.5 1375.0

Transmission/%

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1457.9 2852.2

2923.6

(c) 3349.8

875.5 721.3 813.8 746.3

1693.2 1602.6 1376.9 1461.8 2852.2 2923.6

(d)

3347.9

887.1

1693.2 1600.7

1108.9

813.8 721.3 746.3

1376.9 1461.8

2852.2 2923.6

3500

3000

2000

1500

1000

-1

201

Wave numbers/cm

202

Figure 3. FTIR spectra: (a) parent pitch; (b) PP14 pitch; (c) PP20 pitch; (d) PP25 pitch

203

The distribution of hydrogens among various chemical groupings in the pitches and some

204

suggestive average structural parameters calculated by the modified Brown-Ladner method are

205

listed in Table 4. In comparison with the distribution of constituent hydrogens in starting pitch, the

206

air blowing treatment obviously raises content of aromatic hydrogen Har, followed by decrease in

207

Hα and Hβ contents, suggesting that oxygen attack occurs preferentially at the methyl or methylene

208

sites of the side chains [32]. In addition, it can be also deduced that at the initial stage of air

209

blowing treatment, the generation of the aromatic macromolecules by dehydrogenation of the

210

naphthenic group, dealkylation of alkyl group and successive cross-linking by phenyl-phenyl or

211

methylene bridge among the aromatic constituents [34], is primarily responsible for the growth of

212

softening point in the resultant pitch, which meanwhile leads to rise in the aromaticity index fa and

213

decrease in the condensation index HAU/CA as well as the index of alkyl substitution to aromatic

10


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rings σ (shown in Table 4). When cross-linking degree of derived pitch further deepens, combined

215

with analysis of FTIR spectra, the intermolecular cross-linking by introducing the

216

oxygen-containing functional groups like C=O and C-O-C as the bridge bond occurs to a relative

217

larger extent; such excessive cross-linking reactions always generate a three dimensional,

218

cross-linked molecular configuration prior to the mesophase state [35].

219

Table 4. The distribution of hydrogens among different functionalities in the pitches Weight/% Sample

fa

σ

HAU/CA

19.8

0.28

0.62

0.57

61.6

19.8

0.33

0.52

0.53

12.6

59.5

19.9

0.38

0.44

0.51

12.2

59.4

20.0

0.38

0.42

0.51

Har

Hα

Hβ

Hγ

parent pitch

4.0

13.1

63.1

PP14 pitch

5.8

12.8

PP20 pitch

8.0

PP25 pitch

8.4

220

3.2 Direct Carbonization treatment

221

The direct carbonization behaviors of pitches as well as their PES and PEI fractions,

222

especially in mesophase development and carbonized yield, were evaluated, and the influence of

223

oxidized treatment on direct-carbonization behaviors was expected to be clarified in terms of

224

mutual compatibility of solvent fractions of pitches.

225

3.2.1 Direct carbonization behaviors of oxidized pitches

226

The variations in mesophase content of carbonized residues obtained by direct carbonization

227

at different temperature and soaking time is displayed in Figure 4. The similar change trend of

228

mesophase content with growth of reaction temperatures during the direct carbonization of every

229

pitch can be clearly observed, as presented in Figure 4(a). At the low reaction temperature,

230

mesophase formation and development is restricted because of the slow carbonization rate and

231

high viscosity of reaction intermediate. When the reaction temperature increases up to a point

232

slightly higher than the temperature of initial decomposition, not too many chemical bonds in

233

polycyclic aromatic molecules are in the excited state. Meanwhile this temperature can ensure

234

most polycyclic aromatic molecules react in fluid liquid phase and provide good fluidity of liquid

235

intermediates. Thus, planar aromatic macromolecule with relatively homogeneous molecular

236

structure generates through relatively unitary reaction direction of liquid carbonization induced by

11


Energy & Fuels

237

a certain angle of collision between adjacent molecules. So properly promoting temperature

238

contributes to the generation of those uniform planar macromolecules, parallel stacking of planar

239

macromolecules to form mesophase and subsequent coalescence into mesophase structures of

240

large size. However, as we known, the carbonized process conducted at the too high reaction

241

temperature contributes to rapidly formation of excessive polymerized molecules that are usually

242

loss of molecular planarity and induces fast rise in the viscosity of medium, which significantly

243

inhibit the formation and development of mesophase [22]. As a result, mesophase content

244

increases firstly and then decreases with the growth of reaction temperature, and the optimal

245

temperature for every pitch is 410℃ in terms of mesophase content, as shown in Figure 4(a). 90

Parent Pitch PP14 Pitch PP20 Pitch PP25 Pitch

(a) 4MPa and 6h 80 70

90


70

60 50 40 30 20

60 50 40 30 20 10

10

0

400

246

(b) 410°C and 4MPa

80

Mesophase pitch/vol%

Mesophase comtent/vol%

405

410

415

420

425

430

0

2

T/℃

4

6

8

t/h 85 80

(c) 410°C and 6h

75 70 65

Mesophase content / vol%

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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60 55


50 45 40 35 30 25 20 15 10 1

247

2

3

4

5

Rection pressure / MPa

248

Figure 4. Influence of temperature and soaking time for mesophase content in

249

direct-carbonized residues: (a) variations in mesophase content with different temperatures;

250

(b) variations in mesophase content with different soaking time; (c) variations in mesophase

251

content with different reaction pressure

252

Because constituent molecules of feed stock need to undergo thermal pyrolysis,

12


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253

aromatization and condensation reactions, ultimately to achieve the molecular weight and

254

necessary configuration to generate mesogen molecule. These mesogens leave the parent isotropic

255

phase by the process of ‘self-assembly’, to form mesophase structure that can be recognized in

256

polarizing microscope until its size reach 0.5 µm. Therefore，except the PP25 pitch, the induction

257

period of mesophase formation can be clearly observed in the range between 0h to 2h, seen in

258

Figure 4(b). Owing to high molecular weight and a relatively large amount of oxygen-containing

259

functional groups caused by excessive cross-linked reactions among constituent molecules of feed

260

stock during the air blowing treatment, the phase separation is greatly accelerated during the direct

261

carbonization of PP25 pitch, showing unobvious induction period. After drastic increase of

262

mesophase content from 2h to 6h, the mesophase content turned to mild variations for all the pitch.

263

So the reaction time 6h is selected as optimal soaking time for direct carbonization of every pitch.

264

From Figure 4(c), it can be observed that with reaction pressure increasing from 1MPa to

265

4MPa, the mesphase content is gradually grows, indicating the existence of the high reaction

266

pressure retains more light components in reaction intermediate and lowers the increased rate of

267

viscosity with the carbonization reaction proceeding. While reaction pressure is above 4MPa, too

268

much light component is trapped into liquid intermediate and then hinders the stacking of

269

mesogens and coalescence between mesophase microstructures, causing the mesophase content

270

slightly decreasing. Therefore, the optimal reaction pressure is selected as 4MPa.

271

At their respective optimal reaction conditions of direct carbonization, namely under the

272

condition of 410℃, 4MPa for 6h, the yields of carbonaceous mesophase and their mesophase

273

microstructures consisting of mosaic, small domain, domain and large domain were quantitatively

274

evaluated and results are showed in Figure 5. Therefore, the representative optical textures are also

275

presented in Figure 6.

13


Energy & Fuels

OTI value

(a) 90 80

LD D SD M

70

40

70

20

60

(b) Yield of carbonized residue / wt%

100

0

OTI

Percentage / vol%

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

60

Page 14 of 31

50 40 30 20

Based on blown pitch Based on parent pitch

50

40

30

20

10 10

276

0

Parent Pitch

PP14 Pitch

PP20 Pitch

0

PP25 Pitch

Parent Pitch

PP14 Pitch

PP20 Pitch

PP25 Pitch

277

Figure 5. Variations in (a) optical microstructures distributions and OTI value as well as (b)

278

yields of derived mesophase pitches obtained by direct carbonization of original and

279

oxidized pitches

280

As shown in Figure 5(a), it is found the growth of oxidized degree of treated pitches reduces

281

the mesophase content of the carbonaceous residues, from 82.7% of parent pitch to 24.7% of PP25

282

pitch; by further observing the distributions of mesophase microstructures, it is also seen that the

283

percentage of large domain and domain gradually declines, finally not existing in the resultant

284

mesophase products from direct carbonization of PP20 pitch and PP25 pitch. Additionally, the

285

decrease in the OTI value explicitly shows the oxidized pitches generate more and more poorly

286

developed mesophase with the rise in their softening points, also implying that the air blowing

287

treatment produced adverse effects on the formation and development of mesophase during the

288

subsequent direct-carbonizing processes. Figure 6 displays the optical microscopic images of

289

carbonaceous mesophase derived from direct carbonization under the optimal conditions

290

composed of 410℃, 4MPa for 6h. The brightness in Figure 6 represents optical anisotropy or, in

291

other word, mesophase structures. In Figure 6 (a) and (b), the relatively large sized mesophase

292

structure such as large domain ( length>60, width>10 ) and domain ( size>60 ) can be observed.

293

However, mesophase structure content decreases and large domain mesophase structure disappears

294

in the carbonaceous mesophase obtained from direct carbonization of PP20 pitch ( shown in

295

Figure 6(c) ); and in Figure 6(d) representing the optical image of carbonaceous mesophase from

296

direct carbonization of PP25, the mesophase structure content continuously reduces and only

297

relatively small-sized mesophase structure like mosaic and small mosaic can be visually seen.

298

Those findings also demonstrate that air blowing treatment contributes to the formation of inferior

14


Page 15 of 31

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

299

mesophase during the subsequent direct-carbonizing processes. Nevertheless, according to

300

variations in the yield of carbonized residues showed in Figure 5(b), the increase of oxidation

301

degree effectively serves to the growth of carbonized residue yield from 44.6wt% to 64.5wt%

302

based on air-blown pitch or from 41.8wt% to 58.4wt% based on parent pitch. (a)

(b)

(c)

(d)

303

304 305

Figure 6. Optical microscopic images of mesophase pitch obtained under optimal conditions

306

during the direct carbonization of (a) parent pitch; (b) PP14 pitch; (c) PP20 pitch and

307

(d)PP25 pitch, respectively

308

Just like the scheme oxidized and carbonization reactions shown in Figure 7, because the

309

more and more small-sized molecules in feed which are formerly carried out of reaction system

310

during the carbonization transforms into cross-linked molecules and further forms more stable

311

PAHs molecules, and those molecules more readily remain in carbonized residues, contributing to

312

the increase of the yields of carbonized residues. However, because of aromatization of naphthenic

313

structures and formation of oxygen-containing functional groups during the air blowing process, at

314

early stage of carbonization not only the carbonization reactivity is enhanced due to low thermal

315

stability of oxygen-containing functional groups but also lack of naphthenic hydrogens neutralizes

316

active free radicals, resulting into fast increasing in viscosity of reaction medium and then hinder

317

the formation and coalescence of mesophase structures. In addition, the biphenyl-type structure

318

induced by dissociation of -C=O- or –O-C-O- influences the planarity of PAHs molecule due to 15


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

319

the free rotation C-C single bond to reduce the steric hindrance between aromatic rings [36,37]. As

320

results, the mesophase formation and development of as-prepared pitch are restricted by air

321

blowing treatment.

322 323

Figure 7 Possible scheme of oxidized reactions and carbonization reaction at early stage

324

According to the characterization of average molecular structure discussed in section 3.1, the

325

oxidized treatment has promoted aromaticity fa and reduced the long aliphatic side-chains

326

accompanied by no appreciate incorporation of oxygen atoms, which is generally thought to be

327

beneficial to mesophase development during carbonization [38,39]. But based on the above direct

328

carbonization performance of oxidized pitches, the mesophase development is obviously restricted,

329

indicating that the relationship between average molecular structure and mesophase development

330

excessively ignores interactions between sub-fractions but mutual compatibility between solvent

331

fractions of pitch, namely the carbonization rates and mutual solubility, which plays a critical role

332

in the mesophase development as for the carbonization of oxidized pitches [35,39].

333 334

3.2.2 Relationship between mutual compatibility of sub-fractions and direct carbonization performance of pitches

335

In this section the structural changes of PES and PEI fractions in oxidized pitches and their

336

carbonization performance were also examined to make an attempt to illustrate the relationship

337

between oxidized degree of air-blown pitches and their direct carbonization performance.

338

3.2.2.1 Variations in mutual compatibility between solvent fractions with oxidized treatment 16


Page 16 of 31

Page 17 of 31

339

deepening

340

The variations in respective content of solvent fractions derived from parent and oxidized

341

pitches are presented in Figure 8. The oxidized treatment substantially increases the content of PEI

342

fraction in the derived pitch from 5.9wt% of the parent pitch to 40.5wt% of PP25 pitch at expense

343

of PES fraction from 94.1wt% to 59.5wt%. 100

PEI fraction PES fraction

Content / wt%

80

60

40

20

0

344

Parent Pitch

345

PP14 Pitch

PP20 Pitch

PP25 Pitch

Figure 8. Variations in the content of PES and PEI fractions derived from parent and

346

oxidized pitches

347

The C/H and O/C atomic ratio together with average molecular weight Mw with the air

348

blowing prolonging are shown in Figure 9. As expected, the PES fraction contained the lower C/H

349

atomic ratio, less oxygen content and higher Mw than those of its corresponding PEI fraction. It is

350

noteworthy that there is a more and more obvious gaps in the increase of C/H, O/C and average

351

molecular weight Mw between the PEI fraction and PES fraction with the growth of softening

352

point of the pitch (or air blowing time). 0.90

(a)

PES fraction PEI fraction

40

0.85


(b)

35 30 3

0.80

(O/C)×10

C/H atomic ratio

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

0.75

25 20 15

0.70

10

0.65 5

0

353

5

10

15

20

0

25

5

10

15

Air blowing time/h

Air blowing time / h

17


20

25

Energy & Fuels

12000


(c) 10000

Mw

8000

6000

4000

2000

0 0

5

354

10

15

20

25

Air blowing time /h

355

Figure 9. Variations in (a) C/H atomic ratio, (b) O/C atomic ratio and (c) molecular weight

356

Mw of oxidized pitch obtained at different air blowing time

357

Figure 10 shows the FTIR spectra of solvent fractions derived from PP14, PP20 and PP25.

358

Through the comparison among the FTIR spectra of PES, it is found that almost all peaks lie in

359

2850-3100cm-1, 1600 cm-1, 1455 cm-1, 1380 cm-1and 700-900 cm-1 and no obvious characteristic

360

peaks of oxygen-containing functional groups appear. Whereas O-H stretching peaks at 3350cm-1,

361

C=O stretching peak at 1700cm-1 and phenoxy or ether stretching peak at 1030cm-1 are seen in

362

spectra of PEI derived from PP20 and PP25 pitch, which indicates that oxygen-containing groups

363

prone to concentrate in PEI fractions. Furthermore, the relative absorption intensity

364

Abs1700/Abs1600 and Abs1030/Abs1600 of PEI fraction of PP25 pitch is 0.82 and 0.84 that are higher

365

than 0.64 and 0.67 of PP20 pitch’s PEI fraction. The analysis of FTIR spectra also indicates, to

366

some extent, that more and more oxygen is involved in generation of aromatic molecules of PEI

367

fraction, which is also consistent with the change trend of O/C atomic ratio. (a)

(d)

Transmission/%

Transmission/%

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 31

(b)

(e) 1700

(f)

(c)

1700

4000

368 369

1030

1600

3500

3000

2500

2000

1500

1000

500

4000

3500

3000

2500

2000

1030 1600

1500

1000

500

-1

-1

Wavenumbers/cm

Wavenumbers/cm

Figure 10. FT-IR spectra of solvent fractions: (a) PES of PP14; (b) PES of PP20; (c) PES of

18


Page 19 of 31

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Energy & Fuels

370

PP25; (d) PEI of PP14; (e) PEI of PP20; (f) PEI of PP25

371

Generally, petroleum pitch is regarded as colloidal system in which the PES fraction plays a

372

part in dispersing the PEI fraction and avoiding its coagulation [28]. However, the growing

373

differences aforementioned manifestes that the relatively heavy components of the pitch are prone

374

to be oxidized and then cross-linked into PEI fractions with larger aromatic ring, less number of

375

alkyl groups and the greater number of bridge-bonded oxygen functional group, leaving the light

376

portion of components slightly changed [27, 35]. Those results imply that during air blowing

377

treatment discriminative oxidized reactions continually occurs, leading to poorer and poorer

378

compatibility in carbonization rate and mutual solubility.

379 380

3.2.2.2 Influence of mutual compatibility of sub-fractions on direct carbonization performance

381

The optical microstructure distribution and OTI value of carbonaceous mesophase obtained

382

from direct carbonization of PES fractions and PEI fractions at 410℃, 4MPa for 6h are given in

383

Figure 11. For each pitch, the optical texture of mesophase residues obtained from PES fractions is

384

evidently superior to that from PEI fractions. In addition, it can be also observed that the

385

mesophase content, optical microstructural distributions and OTI values in mesophase residues

386

derived from direct carbonization of PES fractions exhibit relatively mild variations. In contrast,

387

during the direct carbonization of PEI fractions, the obvious and gradual decrease in mesophase

388

content and OTI value occurs with growth of softening point of oxidized pitch. The results are in

389

good agreement with optical microscopic images shown in Figure 12. Via corss-comparison

390

optical texture of mesophase residues produced from direct-carbonization of pitches (described in

391

Figure 5 and 6) and their solvent fractions (described in Figure 8 and 9), it can be concluded that

392

with the growth of oxidized degree of pitches, mesophase development during the direct

393

carbonization of PEI fractions is greatly inhibited and apparently affects the mesophase evolutions

394

of PES fraction during the carbonization of pitch.

19


Energy & Fuels

100

(a)

100

OTI value 50

80

30

50 40

20 30 20

396

OTI

60 50 40 30 20

10

10

10 0

10

LD D SD M

70

40

60

Percentage / vol%

Percentage / vol%

70

395

OTI value

(b) 90

OTI

LD 90 D SD 80 M

0

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 31

Parent Pitch PES

PP14 PES

PP20 PES

PP25 PES

0

0

Parent Pitch PEI

PP14 PEI

PP20 PEI

PP25 PEI

Figure 11 Variations in optical microstructures distributions and OTI value of

397

carbonaceous mesophase obtained at the temperature of 410℃ ℃, reaction pressure of 4MPa

398

for 6h during the direct carbonization of (a) PES fractions and (b) PEI fractions (a1)

(a2)

(b1)

(b2)

(c1)

(c2)

399

400

401

20


Page 21 of 31

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

(d2)

(d1)

402 403 404

Figure 12. Optical microscopic images of carbonaceous mesophase obtained from the condition of 410℃ ℃, 4MPa and 6h during the direct carbonization of solvent fractions:

405

(a1) and (a2) PES and PEI fraction of parent pitch, (b1) and (b2) PES and PEI fraction

406

of PP14 pitch, (c1) and (c2) PES and PEI fraction of PP20 pitch, (d1) and (d2) PES and PEI

407

fraction of PP25 pitch, respectively

408

In conclusion, because there is occurrence of discriminative reactions between the solvent

409

fractions during the air blowing treatment, which enlarges the gap of solubility parameter and

410

carbonization rates between PES fractions and PEI fractions leading to reduce in mutual solubility;

411

during the carbonization process, PEI fractions regarded as highly reactive species because of

412

abundant high-molecular-weight polycyclic aromatics and a high content of heterocycles [41,42]

413

are prone to self-condensation to form macromolecule at the initial reaction stage, which further

414

increase the difference of solubility. Owing to driving force of surface energy minimum, those

415

derived macromolecules precipitate from the matrix resulting in forming isotropic or small size

416

anisotropic microstructures in the following carbonization process, which also reduces the fluidity

417

of the medium and then interfere with the ability of sub-generated planar molecules to diffuse into

418

positions of alignment [40]. Consequently, the mesophase formation and development are

419

restricted due to poorer and poorer mutual compatibility between PES fraction and PEI fraction

420

with the oxidized degree deepening. And the carbonized residue yield is effectively promoted,

421

resulting from rise in both molecular weight that reduces the content of light component and

422

content of oxygen-containing functions that is favorable to intermolecular condensation.

423

3.3 Co-carbonization treatment

424

Although air blowing is effective method to promote the yield of carbonized residue,

425

mesophase development are obviously restricted during the successive direct carbonization. In

426

view of this, co-carbonization treatment was employed, expecting to maintain the yield of

21


Energy & Fuels

427

carbonized residues and improve the mesophase development.

428

3.3.1 Co-carbonization behaviors of pitch-HAO mixtures

429

The influence of temperature, soaking time and amount of HAO additions for mesophase

430

content in co-carbonized residues is presented in Figure 13. It can be clearly seen that change

431

trends of mesophase content induced by different temperature, soaking time or reaction pressure

432

during the co-carbonization are almost the same with that of direct carbonization process shown in

433

Figure 4. As for amount of HAO additions, it can be clearly observed from Figure 13 (c) that when

434

the addition of HAO exceed 20wt% of the pitch, the mesophase content of co-carbonized solid

435

samples are basically no longer change, which demonstrates that 20 wt% addition amount was the

436

point that was enough to fully exhibit the modified effect of HAO. Hence, the optimal reaction

437

condition of co-carbonization is temperature of 420℃, the reaction pressure of 4MPa, the HAO

438

addition of 20wt% and soaking time of 6h for each pitch-HAO mixtures. 100

(a) 4MPa, 6h and 20wt% HAO

90

(b) 420°C, 4MPa and 20wt% HAO

80

80

60

Mesophase content/vol%

Mesophase content/vol%

70

Parent Pitch-HAO PP14-HAO PP20-HAO PP25-HAO

50 40 30 20

60

Parent Pitch-HAO PP14-HAO PP20-HAO PP25-HAO

40

20

10

0 0 400

405

410

439

415

420

425

430

0

2

4

T/℃

6

8

t/h

100

(c) 420°C, 4MPa and 6h

95

90

(d) 420°C, 6h and 20wt% HAO

90


80


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 22 of 31

70

Parent pitch PP14 PP20 PP25

60 50 40

85

80

75

Parent pitch-HAO PP14-HAO PP20-HAO PP25-HAO

70

65

30 60

20 0

440

5

10

15

20

25

30

1

Amount of HAO addition / wt%

2

3

4

5

Reaction pressure / MPa

441

Figure 13. Variations of mesophase content of co-carbonized solid residues obtained under

442

(a) different temperatures; (b) different soaking time; (c) different amounts of HAO

443

additions; (d) different reaction pressure 22


Page 23 of 31

444

According to distributions of optical microstructures and OTI value of co-carbonized

445

mesophase residues obtained at their optimal reaction condition described in Figure 14(a), the

446

domain and large domain microstructures were prevailing in the optical textures of all the resultant

447

mesophase products prepared from co-carbonization with HAO. In comparison with direct

448

carbonization (shown in Figure 5(a)), the co-carbonization of each pitch-HAO mixture markedly

449

increases the mesophase content and OTI values of the carbonaceous residues, strongly implying

450

that adding the HAO facilitates the mesophase formation and development. 70

OTI value 60

(a)

40 20

70

0

OTI

60 50 40 30 20

(b) Yield of co-carbonized residue/wt%

100

LD D 90 SD 80 M Percentage / vol%

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

452

0

50

40

30

20

10

10

451

Based on blown pitch Based on parent pitch

60

Parent Pitch-HAO

PP14-HAO

PP20-HAO

0

PP25-HAO

Parent Pitch-HAO

PP14 Pitch

PP20 Pitch

PP25 Pitch

Figure 14. Distributions of optical microstructures, OTI value and yield of

453

co-carbonized residues derived under the condition of 420℃ ℃, 4MPa, 20wt%of HAO and 6h

454

during the co-carbonization process: (a) optical microstructures and OTI; (b) co-carbonized

455

residue yield

456

Nevertheless, comparing with co-carbonization process of PP25 pitch-HAO mixtures, the

457

parent pitch-HAO mixture, PP14-HAO mixture and PP20-HAO mixture can generate the

458

mesophase residues with high OTI value and the dominant microstructure is domain type,

459

indicating that the co-carbonization modified effect for the oxidized pitches with low cross-linked

460

degree like PP14 pitch and PP20 pitch, is much better than that of excessive cross-linked pitch

461

such as PP25 pitch. Likewise, from observation of optical microscopic image of co-carbonized

462

solid residues shown in Figure 15, it can be seen that the dominating mesophase structure of

463

co-carbonized residues shown in Figure 15(a)(b) is large domain; although the main mesophase

464

structure is still large domain in co-carbonized residues of PP20-HAO mixtures shown in Figure

465

15(c) but domain, small mosaic and mosaic stricture begin to became obvious and in

466

co-carbonized residues of PP25-HAO mixtures shown in Figure 15(d), the large domain display

23


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

467

less while domain, small domain and mosaic become more distinct. This finding is also agreement

468

with the results in Figure 14. Besides, in terms of carbonized yield presented in Figure 14(b), only

469

slight decrease about 0.8-3.0wt% is caused by co-carbonization process comparing with that of the

470

direct carbonization (shown in Figure 5(b)). This finding suggests that combination of air blowing

471

treatment and co-carbonization process could be potentially applied in achieving both high

472

carbonized yields and superior mesophase structure in resultant mesophase products. (a)

(b)

(c)

(d)

473

474 475

Figure 15. Optical microscopic images of the resultant products derived under the

476

condition of 420℃ ℃, 4MPa, 20wt%of HAO and 6h during the co-carbonization of the feeds:

477

(a) parent pitch; (b) PP14 pitch; (c) PP20 pitch; (d) PP25 pitch

478

3.3.2 Effect of oxidized degree of pitches on modified effect of HAO during co-carbonization

479

The 1H-NMR characterization and oxygen content in co-carbonization feed are presented in

480

Table 5. It can be found that the oxygen content in PP14-HAO, PP20-HAO and PP25-HAO

481

mixtures decreases in contrast with growth in parent pitch-HAO, but the amplitude of all

482

variations is very slight in comparison with oxygen increments induced by air blowing with

483

prolonged blowing time (shown in Table 3), implying that the adding HAO in co-carbonization

484

feed hardly causes obvious influence on oxygen content of mixtures. Additionally, based on

24


Page 24 of 31

Page 25 of 31

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

485

cross-comparison between Table 4 and Table 5, the increase in Har and decrease in Hα and Hβ

486

content in accordance with increase in fa, reduction in both σ and HAU/CA clearly shows the

487

mixtures become less paraffinic, suggesting the aromatic hydrogen-transfer reactions may be

488

promoted during the co-carbonization; combined with the distinct high naphthenic hydrogen ( HN )

489

content shown in Table 1 indicating the abundant naphthenic structures attaches to aromatic rings

490

in constituent molecules of HAO, it can also be assumed that naphthenic hydrogen-transfer

491

reactions largely occurs. Based on above discussions, the mesophase improvement during

492

co-carbonization mainly attributes to the aromatic and naphthenic hydrogens-transfer reactions not

493

the dilution effect of oxygen-containing functional groups. Table 5. 1H-NMR characterization and oxygen content variations in co-carbonization

494 495

feed Weight/% Sample

496

fa

σ

HAU/CA

Oc/wt%

Har

Hα

Hβ

Hγ

parent pitch-HAO

6.6

15.3

58.1

20.0

0.32

0.54

0.66

0.75

PP14-HAO

8.0

15.1

56.9

20.0

0.35

0.49

0.63

1.16

PP20-HAO

9.8

14.9

55.3

20.1

0.38

0.43

0.61

1.37

PP25-HAO

10.1

14.6

55.2

20.1

0.39

0.42

0.60

1.65

c

refers to calculated oxygen content by formula as followed: Oc / wt%=Opitch×80%+OHAO×20%;

497

In order to further illustrated the role of HAO during the co-carbonization process, the

498

distribution of solvent fractions of intermediates prepared from carbonization at the soaking time

499

of 2h and their respective optimal temperature are listed in Table 6. It is need to be pointed out that

500

all feedstock including pitches and their mixtures with HAO dissolves well in toluene, and no

501

toluene insoluble (TI) were generated when the HAO are carbonized alone at reaction pressure of

502

4MPa and the temperature of 420℃for 2h. Therefore, it can be concluded that TI is mainly

503

derived from reactions of reactive species in PEI fractions. From Table 5, unlike direct

504

carbonization process, the pyridine insoluble (PI) no longer formed during the co-carbonization

505

process of parent pitch-HAO mixtures, PP14-HAO mixtures and PP20-HAO mixtures; and the

506

resultant content of PI substantially reduced from 18.7wt% to 5.4wt%. These variations manifests

25


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

Page 26 of 31

507

that adding HAO can lower the reaction rate of highly reactive species during the co-carbonization

508

process.

509

Table 6. Distributions of solvent fractions in samples prepared by carbonization at 2 h, 4

510

MPa Weight% Feedstock TS

TI-PS

PI

parent pitch

68.7

27.2

4.1

parent pitch-HAO

85.8

14.2

0

PP14 pitch

60.2

33.2

6.6

PP14 pitch-HAO

68.2

31.8

0

PP20 Pitch

50.1

39.6

10.3

PP20 Pitch-HAO

54.3

45.7

0

PP25 Pitch

40.8

40.5

18.7

PP25 Pitch-HAO

39.9

54.7

5.4

511

Combined with structural analysis of HAO shown in Table 1, the high aromaticity and

512

naphthenic structure attached to aromatic rings are the structural feature of HAO. So it can be

513

deduced that the additive HAO, as a part of PES fraction in the pitch-HAO mixtures, can induce

514

partial hydrogenation and condensation without extensive dehydrogenation of highly reactive

515

species by means of HN-transfer reactions and provide relatively similar solubility parameters with

516

the free radical generated from constituent molecules of PEI fraction to achieve dispersion of

517

highly reactive species or its free radical [38,40,41,42]. So these polycondensed molecules formed

518

at the initial stage of carbonization exhibit good mutual solubility with the matrix and then

519

participate in the formation of well-developed mesophase in the subsequent process. In addition to

520

improving the formation of mesogens, both HN-transfer reactions and dilution effect can increase

521

the fluidity of the carbonization medium, which is also a critical factor to generate extensive

522

developed mesophase during the co-carbonization.

523

However, the three-dimensional molecular conformations, predominantly involving in

524

petroleum ether-insoluble fractions of PP25 pitch, were substantially generated, mainly by

525

introducing the appreciable amount of oxygen-containing functional groups. Thus, adding HAO

26


Page 27 of 31

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Energy & Fuels

526

still fails to contribute to the bridge bond cleavage and then be restructured into the planar

527

macromolecules with the low viscosity of reaction medium [16]. Hence, the cross-linking degree

528

of the treated pitch should be controlled so as to obtain both excellent mesophase and high carbon

529

yield of carbonaceous mesophase during the co-carbonization process.

530

To sum up, the additive HAO improves the mutual compatibility of carbonization between

531

the solvent sub-fractions, that is, by providing the necessary physical fluidity of the system and

532

possible some chemical stability via the dilution effect and hydrogen transfer reactions.

533

4. Conclusion

534

Pitches with different oxidized degree produced carbonized residues with different

535

mesophase

development

during

the

direct

carbonization

and

co-carbonization

with

536

hydrogen-donor aromatic oil (HAO), which could be explained from variations in the mutual

537

compatibility of solvent fractions during the carbonization as well as macromolecular structure

538

induced by the cross-linking treatment.

539

Despite of significant rise in carbon residue, the air blowing treatment for the pitch exhibited

540

adverse effect on the mesophase development during the subsequent direct carbonization of the

541

treated pitches, which is ascribed to enlargement in the difference in reactivity and mutual

542

solubility of PES and PEI fractions generated during the cross-linked treatment. Also, during the

543

early stage of subsequent carbonization, the loss of molecular planarity in mesogens such as

544

formation of biphenyl structure induced by dissociation of oxygen-containing functional groups

545

restricts the formation and development of mesophase.

546

The hydrogen-donor aromatic oil (HAO) as a portion of PES fraction of the pitch-HAO

547

mixtures effectively improves the mutual compatibility of the solvent fractions via H-transfer

548

reactions and dilution effects during the co-carbonization process. However, the three dimensional

549

molecular conformations in PP25 pitch caused by excessive cross-linking among the components

550

still fails to be transformed into the planar macromolecules with the low viscosity of reaction

551

medium during the co-carbonization process.

552

■ ASSOCIATED CONTENT

553

Supporting Information: Selection of conditions during the air blowing treatment, detailed

554

description of carbonization experimental procedure and experimental design

555

■ ACKNOWLEDGMENTS 27


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

556

The authors gratefully acknowledge financial support from the Fundamental Research Funds for

557

the Central Universities (15CX05009A), Shandong Provincial Key Project of Research and

558

Development, China (2016GGX102017) and Shandong Provincial Natural Science Foundation,

559

China (ZR2015BM003).

560

Notes

561

The authors declare no competing financial interest.

562

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Structural Modification of Petroleum Pitch Induced by Oxidation

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