Preparation of the Mesophase Pitch by Hydrocracking Tail Oil from a

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Preparation of Mesophase Pitch by Hydrocracking Tail Oil from Naphthenic Vacuum Residue Ming Li, Dong Liu, Renqing Lv, Jiashun Ye, and Hui Du Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00537 • Publication Date (Web): 22 Jun 2015 Downloaded from http://pubs.acs.org on June 23, 2015

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Preparation of Mesophase Pitch by Hydrocracking Tail Oil from Naphthenic Vacuum Residue Ming Li, Dong Liu*, Renqing Lv, Jiashun Ye, Hui Du State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China ABSTRACT: The raw oils P1, P2 and P3 with different properties from naphthenic base vacuum residue were used for preparing mesophase pitch through high pressure thermal treatment. In the first,

1

H-NMR, FI-IR and VPO were employed to

characterize the structural parameters of the raw materials. Development of the anisotropic mesophase was monitored by polarised light optical microscopy. Four fractions HS, HI-TS, TI-PS and PI of the product were obtained by extracting with heptane, toluene and pyridine. The molecular and crystal structure was analyzed by FI-IR, 1H-NMR and XRD. The effects of the feed composition on the formation of mesophase pitch were investigated. The results showed that the structure of the raw material play a very important role on mesophase pitch formation. Moreover, the raw material with higher aromatic, more naphthenic structure and a certain amount of short alkyl side chains was easy to form mesophase pitch with larger domains optical texture, lower softening point and better crystal structure. KEYWORDS: mesophase pitch; naphthenic base vacuum residue; optical texture; molecular structure; crystal structure

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1. INTRODUCTION Mesophase pitch is composed of flat disk-like polycyclic aromatic hydrocarbons which is prepared from bitumen, heavy oil, coal tar and other feedstock by thermal treatment or from aromatic compounds through catalytic condensation.1 Mesophase pitch is widely recognized as excellent precursor for making carbon fibers, needle coke, carbon electrode material, foam materials and other advanced function materials.1-4 As previously reported, the number of alkyl group in raw material was one of the most important factors to control the solubility and the anisotropic content of mesophase pitch.5-7 The effect of the alkyl groups on the formation of mesophase pitch has been studied. The research exhibited that the combination of the hydrogenation and alkylation was quite effective method to improve the properties of mesophase pitch.6-9 The naphthenic groups were introduced into the mesophase pitch to further improve the polarized property through hydrogenation, and the solubility during pyrolysis or condensation polymerization was also greatly improved simutinously.10,11 Miyake et al have tried to assess the effects of alkyl groups and hydrogens on the formation of anisotropic texture.12 In their work, a mesophase pitch, which contained only a few alkyl groups per molecule, was treated by reductive alkylation (methyl and ethyl) and hydrogenation to study the effects of reductive alkyl groups and hydrogens on anisotropic texture formation. When the hydrogenated pitch was carbonized, the anisotropic content grew with the increase of the number of introduced hydrogens, while the anisotropic content varied depending on the number and steric size of introduced alkyl groups. After studying the preparation of naphthalane and methyl naphthalane mesophase pitches, Mochida et al13-16 found that hydrogenation and reductive alkylation methods


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are effective methods to convert the quinoline insolubles of pitches into graphitizable carbon. Yamada17 and Oyabu18 reported that the coal tar pitch could be used for preparing the spinnable mesophse pitch with tetrahydroquinoline as hydrogen donor. Compared with untreated coal tar pitch, the softening point (SP) was decreased and aliphatic carbons were increased after adding tetrahydroquinoline. Korai et al19 suggested that the methyl on the mesophase molecule is the main influence factor of mesogen molecules stacking. Yoon et al also proposed that the number of methyl groups have an influence on the properties of the mesophase pitch. The greater number of methyl groups in C9 CNOOC petroleum pitch decrease the softening point and increased the bulking thickness of mesogen molecules.20 In conclusion, the influences of the alkyl side chain and naphthenic group on the optical property and molecular structure of mesophase pitch are worth to be studied comprehensively. Here in this work, three batches of raw materials (P1, P2 and P3) with different properties were derived from naphthenic hydrocracking tail oil and then mesophase pitch was prepared by thermal treatment, namely P1-MP, P2-MP and P3-MP. The influences of the raw materials on the properties of mesophase pitch were studied, and the carbonization mechanism discussed.

2. EXPERIMENTAL SECTION 2.1 Materials Three batches of raw materials (P1, P2 and P3) with different structures are derived from naphthenic hydrocracking tail oil. 2.1.1 Material compositions The characteristics of the three raw materials are listed in Table 1. From table 1, all of the tested samples contained no asphaltene, the order of aromatics content was as follows: P3＞P2＞P1. However, the resin content increased with the order of P2＞P1


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＞P3. Additionally, the relative molecular mass, n (H): n (C) and the coking coke ratio decreased as P3＜P2＜P1. Table 1 Basic properties of feedstocks Sample

wt/%

wt/%

n(H):

M

Coking

Saturates

Aromatics

Resin

Asphaltene

C

H

n(C)

coke/wt%

P1

30.15

56.40

13.45

0

90.08

7.65

1.0191

310.12

1.79

P2

6.49

77.34

16.17

0

90.25

7.43

1.0034

299.87

0.43

P3

8.51

82.67

8.82

0

90.12

7.49

0.9973

294.53

0.37

M, Relative molecular mass.

2.2 Thermal Treatments The raw material was placed in the 300ml high-pressure agitated autoclave and heated in an electric furnace to 435-445℃ with a rate of less than 3℃/min after purging the reaction system by nitrogen for 3 times. During the experiment, the pressure was maintained at about 4MPa. After the pyrogenation process at 435-440℃, the reaction was carried out under 4 MPa for 2-12 hours. 2.3 Analysis SARA compositions of raw materials were measured according SH/T 0509-98 standards. Elemental analyses of samples were carried out by the Chnos company, Vario EL II, Germany. The relative molecular mass of the raw materials were obtained on German Knauer company Vapour Pressure Osmometer K-700 molecular weight apparatus. FT-IR analysis was performed on a Nicolet FT S215 FT-IR spectrometer to determine the functional groups. The toluene-soluble fraction of raw materials and the pyridine-soluble fractions of mesophase pitches were determined by 1H-NMR on a US Varian company Unity-200MHz FT NMR spectrometer using DCCl3 as the solvent and tetramethylsilane (TMS) as the internal standard. The operating frequency


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is 80MHz, and the scanning width is 2 KHz. Microstructures were observed and photographed by BX51-P polarizing microscope with camera (made in Germany). The mesophase content was determined by polarized optical microscopy. The images taken by the microscope (with either cross-polarizer or parallel-polarizer setup) were analyzed to measure their brightness which was proportional to the intensity of the (perpendicular or parallel components of) backscattered light. The crystal structure of the mesophase pitch was characterized by a PANalyitcal X Pert PRO MPD X-ray diffraction (XRD) with a Cu Kα radiation (λ = 0.15418 nm). The group composition of mesophase pitch was analyzed by the solubility in n-heptane, toluene and pyridine.21 The specific procedures were as follows. All mesophase pitches were ground to ＜500µm particle size. Six gram samples of mesophase pitch were extracted, quantitatively, in a Soxhlet apparatus with n-heptane (H), toluene (T) and pyridine (P), at their boiling points. Initially, the pyrolysis product was separated into n-heptane-soluble fraction (HS) and n-heptane-insoluble fraction (HI). The n-heptane-insoluble/ toluene-soluble fraction (HI-TS) and toluene-insoluble fraction (TI) were acquired from the extraction of HI with T. Additionally, the toluene-insoluble/ pyridine -soluble fraction (TI-PS) and pyridine -insoluble fraction (PI) were obtained from the extraction of TI with P. The coking coke of raw materials and mesophase pitches were analyzed according to GB/T 8728-2008.

3. RESULTS AND DISCUSSION 3.1 FI-IR Analysis 3.1.1 FT-IR analysis of the raw materials


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P1

P2 P3

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber/cm

Figure 1 FI-IR spectra of raw materials

The FT-IR spectra of the raw materials are shown in Figure 1. In the aliphatic hydrocarbon absorption region, the absorption peaks at 1460cm-1, 2853cm-1, 2923cm-1 are the characteristic bands of -CH2- and 1380cm-1, 2960cm-1 are the characteristic peaks of -CH3. These absorption peaks can be observed in the IR profile of three kinds of raw materials in Figure 1, indicating that the raw materials contain many alkyl side chains.22 The molar ratio of -CH2- (absorption peaks at 1460cm-1) to -CH3 (absorption peaks at 1380cm-1) was evaluated according to the following formula.23

r = 3.07

A1460 − 3.72 A1380

(1)

In the formula, A1460 and A1380 represent absorption peak intensities at 1460cm-1 and 1380cm-1, respectively. The results showed that the rates of P1, P2 and P3 were 3.44, 2.01 and 1.52, suggesting that the alkyl side chain length of the three batches of raw materials became shorter successively. The absorption peaks at 1460cm-1, 1600cm-1 are the bands of aromatic hydrocarbon. In the condensed aromatics absorption region,


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the bands of polycyclic aromatic hydrocarbons are at 742cm-1, 803cm-1 and 867cm-1. The aromaticity (fa) of the raw material was calculated according to the following formula.23

fa = 0.574P + 0.024

(2)

In the formula, P is defined as A1600 / (A1600 +0.16 A1460 + 0.23A1330), and A1660 is 1660cm-1 absorption peak intensity. The aromaticity (fa) of P1, P2 and P3 were 0.39, 0.41 and 0.42 respectively, implying that the aromaticity of the three batches of raw materials increased. The changing tendency of aromaticity matched that of aromatics content. 3.1.2 FT-IR analysis of mesophase pitch The FT-IR spectra of P1-MP-8h (mesophase pitch prepared from P1 for 8 hours), P2-MP-8h (mesophase pitch prepared from P2 for 8 hours) and P3-MP-8h (mesophase pitch prepared from P3 for 8 hours) are shown in Figure 2. Compared with the raw materials, the aliphatic hydrocarbon absorption intensities of P1-MP-8h, P2-MP-8h and P3-MP-8h increased with the decreasing of condensed aromatics absorption, which suggested a thermal cracking of aliphatic chains and a polycondensation of molecules.


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P1-MP-8h

P2-MP-8h

P3-MP-8h

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber/cm

Figure 2 FT-IR spectra of the mesophase pitches

As shown in Figure 2, the weak absorption peaks at 3019, 1150 and 1033cm-1 are ascribe to C=O stretching vibrations, S-H out of plane vibrations and N-H stretching vibrations, which illustrate that the heteroatom are not completely removed during the carbonization. The FT-IR peaks at 2920, 3040 and 1380cm-1 are the stretching vibration of alkyl C-H, stretching vibration of aromatic C-H and symmetrical vibrations of -CH3, indicating that a certain amount of aliphatic side chains are contained in the mesophase pitch.22 The aromatic indexes (Iar) of P1-MP-8h, P2-MP-8h and P3-MP-8h calculated by the following formula23 were 0.750, 0.734 and 0.720, demonstrating that the contents of aliphatic groups of mesophase pitches grew with the increase of naphthenic groups and the shorten of alkyl side chains in raw materials. I ar =

Ab3040 Ab3040 + Ab2920


(3)

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The 880/1600cm-1 peak intensity ratio (isolated aromatic C-H/aromatic C=C) were used to reveal the condensation degrees of polycyclic aromatic hydrocarbons (PAH).23 And the PAH of P1-MP-8h, P2-MP-8h and P3-MP-8h were 1.548, 1.484 and 0.939, which showed that the condensation degrees of P1-MP-8h, P2-MP-8h and P3-MP-8h decreased respectively. The changing tendency of PAH was consistent with that of aromatic indexes (Iar). 3.2 1H-NMR Analysis of Mesophase Pitch 3.2.1 1H-NMR analysis of the raw materials

Figure 3 1H-NMR spectra of the raw materials

Table 2 Structural parameters of the raw materials Sample

Har

Hα

Hβ

Hγ

Hn

fa

L

P1

33.74

34.82

16.48

8.98

5.99

0.38

4.55

P2

33.20

27.87

13.07

9.09

16.77

0.40

1.95

P3

32.25

14.57

6.49

11.52

35.17

0.42

0.39

Har, aromatic hydrogens; Hα, aliphatic hydrogens in methyl or methylene groups in α-position to an aromatic ring (3.3-2.0 ppm); Hn, naphthenic hydrogen (2.0-1.4 ppm); Hβ, aliphatic hydrogens in methyl or methylene groups in β-position to an aromatic ring (1.4-1.0 ppm); Hγ, aliphatic hydrogens in methyl or methylene groups in γ-position to an aromatic ring (1.0-0.5 ppm); fa, aromaticity; L, Average alkyl chain lengths.23-24


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The 1H-NMR spectra of raw materials are shown in Figure 3 and the analysis results are shown in Table 2. It revealed that the Hα content of P1, P2 and P3 decreased, meanwhile, the Har and Hγ contents increased gradually, indicative of increased amounts of aliphatic side chains. The aliphatic side chain could reduce the reaction activation energy effectively and increased the reaction rate by producing active radicals.24-27 The branching degree (Hγ/Hβ) of P1, P2 and P3 were 0.55、0.69 and 1.78 respectively, which implied the elongation of their alkyl side chains.

23, 28

The

proportion of Hn in P1, P2 and P3 were 5.99, 13.77 and 35.17, illustrating that the naphthenic structures contained in P1, P2 and P3 increased successively. Hydrogen transfer reactions of napthenic structures that occur during pyrolysis could reduce the reactivity of free radicals, maintain the flowability and increase the solubility of the mesophase pitch.24, 29 As a result, it is easier to obtain a mesophase pitch with large domain optical texture and good crystal structure. All the analyses above showed that a certain amount of naphthenic groups and alkyl side chains contained in the raw material were conducive to the formation of mesophase pitch. But if the raw material contained excessive amounts of long alkyl side chains and alkyl substituents, it would produce more free radicals, which make the polycondensation reaction very fast and hinder formation of a mesophase phase with low softening point. 6, 8 So molecules in raw materials should contain a certain number of naphthenic groups and short alkyl side chains in order to get a mesophase phase with large domain optical texture and highly crystalized structure. The structural parameters of the raw materials shown in Table 2 were calculated according to the improved Brown-Ladner method.23 The aromaticity (fA) of P1, P2 and P3 calculated by the formula 4 were 0.38, 0.40 and 0.42, which was consistent


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with the FT-IR analysis results. The average alkyl chain lengths (L) of raw materials were calculated by the formula 5 according to the improved Brown-Ladner method.23

fA =

L =

C / H − (H α + H β + H γ ) / 2(H ∂r + H α + H β + H γ ) C /H

CP N CH

N CH = 3

N CH N CH

2

3

(5)

3

CS 2 + N CH 2 / N CH 3

= 2.93

(4)

A1460 − 3.70 A1380

(6)

(7)

In the formula 5, Cp is the alkyl carbon number of the molecule; Cs is the naphthenic carbon number of the molecule; NCH3 is calculated according to the formula 6 and 7. 23

The average alkyl chain lengths (L) of P1, P2 and P3 were 4.55, 1.95 and 0.39. The

short alkyl side chain could improve the reaction rate to a certain extent. So it was beneficial to the formation of excellent mesophase pitch with large domain optical texture and highly crystalized structure. 3.2.2 1H-NMR analysis of the raw materials

The pyridine-soluble fractions of P1-MP-8h, P2-MP-8h and P3-MP-8h (P1-MP-8h -PS, P1-MP-8h -PS and P1-MP-8h-PS) were analyzed by 1H-NMR. The 1H-NMR spectra of the pyridine-soluble fractions are shown in Figure 4 and the distribution of the hydrogen atoms are tabulated in Table 3.

Figure 4 1H-NMR spectra of MP-PS


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Table 3 Hydrogen atoms distribution of MP-PS Sample

Har,%

Hα,%

Hβ,%

Hγ,%

Hn,%

P1-MP-8h-PS

86.36

6.14

2.81

1.64

2.89

P2-MP-8h-PS

79.61

12.09

3.78

1.55

2.97

P3-MP-8h-PS

74.56

15.32

5.92

1.31

3.05

As shown in Table 3, the Har contents of P1-MP-8h-PS, P2-MP-8h-PS and P3-MP-8h-PS increased, while the Hβ and Hγ contents decreased gradually compared with the raw materials. It implied that the pyrolysis process was mainly to remove Hβ and Hγ, and their removal resulted in the generation of aromatic intermediates which were stable radicals. In addition, the steric hindrance of the aromatic intermediates containing α hydrogens decreased, which made it easier for polymerization of large aromatic molecules when the aromatic radicals combined with each other. This kind of structure was more suitable for the ordered accumulation of aromatic layers. The alkyl side chains of mesophase pitch were mainly α and β substituents, which indicated that there was a certain amount of CH3- and -CH2- in position α and β of aliphatic substituent groups. The weak absorption peaks of -CH2- (at 3.3-4.2) in Figure 4 indicated that the generated aromatic layers were connected with each other through -CH2- bonds.30-31 These low condensation degree molecules were favorable to the interpenetration of carbon layers and rearrangement during the growth of mesophase spherule, so it was easier to produce network molecule with no or less internal defects and generate mesophase pitch with ordered molecular structure.6,30 The aromaticity (fa) of P1-MP-8h-PS, P2-MP-8h-PS and P3-MP-8h-PS calculated by n-d-M method23 were 0.788, 0.738 and 0.693, implying that the aliphatic groups of


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the three products increased as the order: P3-MP-8h-PS ＞ P2-MP-8h-PS ＞ P1-MP-8h-PS. 3.3 Optical Textures Analysis of Mesophase Pitch The optical micrographs of P1-MP, P2-MP and P3-MP are shown in Figure 5, 6 and 7. As can be seen in Figure 5, a number of small spheres and a few of ball aggregates were formed from the mother liquor of the mesophase pitch after 4 hours at a certain temperature. A mesophase pitch with mosaic structure was generated via thermal polycondensation after 6 hours. With the development of reaction, it was observed that the mesophase pitch with medium domain structure diameter in 100µm~200µm on average after 10 hours. Moreover, the softening point of P1-MP-8h was 289℃, and the coking coke was 74.6%. When the carbonization time was too long (over 12 hours), a mesophase pitch with fine mosaic structure was formed, because the over-carbonization destroyed the crystal structure.

100µm

100µm

100µm

100µm

100µm

100µm

Figure 5 Optical micrographs of P1-MP

Due to the high condensation degree of P2, it just needed 4 hours to obtain a mesophase pitch with mosaic structure through thermal polycondensation (in Figure


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5). It is observed that a mesophase pitch with large domain structure (the diameter were around 200µm) appeared after 8 hours. The softening point of P2-MP-8h was 251℃, and the coking coke was 69.8%. This domain structure were retained after 10 hours, however, a mesophase pitch with coarse mosaic structure was formed because of the overreaction.

100µm

100µm

100µm

100µm

100µm

100µm


Because the condensation degree of P3 was the highest, a mesophase pitch with a mixed structure of mosaic and domains structure appeared after 4 hours as shown in Figure 7. With the deepening extent of reaction, a mesophase pitch with large domain optical structure appeared after 6 hours and the diameter was still larger than 200µm after 10 hours. Both coarse mosaic and fine domains structure were mixed together in the polarized light microscopic picture of mesophase pitch when coking for 12 hours. The softening point of P3-MP-8h was 234℃, and the coking coke was 68.2%. The results showed that the order of softening points was as follows: P1-MP-8h ＜


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P2-MP-8h＜P3-MP-8h，which was consistent with the change of their coking coke. Although the coking coke of mesophase pitch declined, the domain optical structure in the optical picture was improved. In the future research, the coking coke of mesophase pitch will be improved by adding additives.

100µm

100µm

100µm

100µm

100µm

100µm


In summary, P1 leads to a produced mesophase pitch with medium domain structure after 10 hours, while the mesophase pitches produced from P2 and P3 have large domain structure (8 h and 6 h, respectively). But the polarized light microstructure of P3-MP is more ordered than that of P2-MP. The results show that only P2 and P3 can generate mesophase pitch with large domain optical texture under certain conditions through carbonization. The possible reason is that P1 contain less naphthenic groups and more alkyl side chains, while P2 and P3 have more naphthenic groups and less alkyl side chains. Moreover, the alkyl side chains of P2 and P3 are shorter than that of P1. The alkyl side chains of P1 can be decomposed quickly to produce active radicals,


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so that the reaction activation energy is reduced effectively and the reaction rate is increased, then the viscosity of the whole system increase quickly.6,36 As a result, the growth and coalesce of mesophase sphere are hindered. Therefore, P1 cannot be converted to a mesophase pitch with large domain optical texture. However, P2 and P3 contain more naphthenic groups which lead to hydrogen transfer reaction during the pyrolysis process. The hydrogen transfer reaction plays a role in easing the carbonization, as a consequent, the reaction system maintains low viscosity and the surface energy of sphere is reduced, then the mesophase spheres are easy to coalesce and form mesophase pitch with ordered polarized light microstructure.11,36 3.4 Group Composition Analysis of Mesophase Pitch The yields-time curves of HS, HI-TS, TI-PS and PI in mesophase pitches are shown in Figure 8.


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100

P1-MP

Yield (wt%)

80 60 HS HI-TS TI-PS PI

40 20 0 0

2

4

6

8

10

Time (h)

100 HS HI-TS TI-PS PI

P2-MP 80

Yield (wt%)

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 40 20 0 0

2

4

6

8

Time (h)


10

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100 HS HI-TS TI-PS PI

P3-MP 80

Yield (wt%)

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 40 20 0 0

2

4

6

8

10

Time (h)

Figure 8 Variation of HS, HI-TS, TI-PS and PI yields with time

Generally, carbonization reaction is a continuous reaction. As shown in Figure 8, the yields of HS and PI increased with the proceeding of carbonizing treatment, while the yields of HI-TS and TI-PS increased at first and then decreased. The reason was that HS-TI and TI-PS were the intermediate products and its yields changed depending on its production rate and consumption rate.16-17,32 The results showed: 1. The main reaction was the cracking of alkyl side chains to increase their aromaticity at the preliminary stage of pyrolytic reaction. The raw material P1 contained longer alkyl side chains, therefore the yield of HS decreased rapidly. Only when the aromatic degree reached a certain level, the solubility of the molecules would change. 2. During the process of thermal treatment, the yields of intermediate components HI-TS and TI-PS in P1-MP, P2-MP and P3-MP increased at first then decreased, respectively. The maximum values of HI-TS and TI-PS were largest in the curve of P1-MP and became smallest in the curve of P3-MP. It implied that the intermediate component yields of P1-MP, P2-MP and P3-MP changed slowly


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and the thermal polycondensation reaction was restrained during the process of thermal treatments. The reason may be that the hydrogen transfer reaction took place owing to the naphthenic groups which could produce macromolecular free radicals to restrain the polycondensation reaction. Thereby the system viscosity increased slowly. The lower system viscosity could help fulfill the transformation of each component, so the intermediate components were converted to the larger molecular components rapidly once they were generated. It was beneficial to generate the mesophase pitch with homogeneous molecular structure (molecular weight distribution was more concentrated). 3. When the pyrolytic reaction is taken over, the PI yields of P1-MP, P2-MP and P3-MP decrease gradually. 3.5 Solublility of Mesophase Pitch Mesophase pitch comprises soluble mesophase with lighter molecules and insoluble mesophase with heavier molecules.33 Generally, the soluble mesophase contents is the difference between the contents of anisotropic fraction and pyridine-insoluble fraction. The soluble mesophase content is one of the key factors influencing the properties of mesophase pitch. It determines the fluidity and viscosity of mesophase pitch.34, 22 In that sense, soluble mesophase is one of the crucial performance indexes of mesophase pitch.35 Therefore, the anisotropic fraction, pyridine-insoluble fraction and soluble mesophase contents of mesophase pitches are analyzed and shown in Table 4. The polarized light microstructures of the mesophase pitches are shown in Figure 5, 6 and 7. Table 4 Anisotropic fraction, pyridine-insoluble fraction and soluble mesophase contents of mesophase pitches

Anisotropic, %

P1-MP-8h

P2-MP-8h

P3-MP-8h

P3-MP-12h

89

100

100

100


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PI, %

61

51

47

69

S-meso, %

28

49

53

31

PI, pyridine-insoluble fraction; S-meso, Soluble mesophase; P3-MP-12h, mesophase pitches prepared from P3 after 12 hours.

Table 4 showed that the soluble mesophase contents of three products increased as the following order: P1-MP-8h < P2-MP-8h < P3-MP-8h (28%, 49% and 53%, respectively). The changing tendency of their anisotropic contents were consistent with that of soluble mesophase contents, the anisotropic content of P1-MP-8h was 89% while P2 and P3 formed a completely coalescence mesophase pitch. When P3 was carbonized for 12 hours at a temperature, a completely coalesced mesophase pitch (named P3-MP-12h) with coarse mosaic optical texture was prepared. The soluble mesophase content of P3-MP-12h was 31%, which indicated that the content of soluble mesophase pitch decreased owing to over-carbonization. Additionally, the polarized light microstructure of P3-MP-12h became obviously worse than that of P3-MP-8h. All the analysis showed that the soluble mesophase content elevated with the increase of raw materials’ aromatic, shorten of alkyl side chains and increase of naphthenic groups. Good fluidity of the reaction system could accelerate the coalescence of mesophase pitch, so the optical texture of mesophase pitch was improved obviously. 3.6 The XRD Analysis of Mesophase Pitch The XRD spectra of mesophase pitches are shown in Figure 9 and the X-ray data of are tabulated in Table 5. Three kinds of mesophase pitches all exhibited strong diffraction peaks as shown in Figure 9, which indicated that the mesophase pitches were highly crystalized. The X-ray data (shown in Table 5) revealed that the stack


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height (Lc) of the mesophase pitches increased and the interlayer spacing (d002) decreased with the increase of naphthenic groups and the shorten of alkyl side chain in the raw materials. At the same time, the alignment degrees (Og) of the mesophase pitches were improved. The solubility of mesophase molecules in an isotropic matrix was determined by the distribution of mesophase molecule structures, and the movement of mesophase molecules was determined by the solubility of mesophase molecules. Thereby it affected the orientation of the aromatic layer in mesophase pitch.

P1-MP P2-MP P3-MP

0

10

20

30

40

50

60

70

80

0

Position [ 2theta]

Figure 9 XRD spectra of the mesophase pitches

Table 5 X-ray parameters of the mesophase pitches Code

d002(nm)

Lc(nm)

Og

P1-MP

0.3471

2.4

0.9062

P2-MP

0.3461

3.1

0.9691

P3-MP

0.3451

3.8

0.9802


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During the carbonization process, the produced free radicals can accelerate the crosslinking of intramolecular and intermolecular. Compared with P2 and P3, P1 contains more alkyl side chains which would produce more free radicals, resulting in the increase of viscosity rapidly through condensation. The anisotropic structure is formed when the size of molecular reaches certain level. The content of naphthenic groups increase in the order of P1 < P2 < P3, while length of alkyl side chain decreased from P1 to P3. The large content of naphthenic groups lead to hydrogen transfer reaction which plays a role in easing the carbonizing reaction, so that the viscosity of the system is lowered and then the mesophase pitch molecules have enough time to rearrange. Therefore, the optical texture of P1-MP presents in small domains and the optical texture of P3 mostly is revealed in everywhere. The interaction of mesophase molecules is larger than that of solvent molecules. Generally, the interaction of mesophase molecules is reduced significantly due to the increase of alkyl substituent, and thus it can approach the function of solvent molecules. It is implied that the interaction between mesophase molecules and solvent molecules become stronger. Therefore, the contents of soluble mesophase increase. Consequently, the softening points of mesophase pitches decline and the fluidity of the products is enhanced, which is beneficial to the coalescence of mesophase pitch. Moreover, the orientated mesophase pitch is able to be obtained during the process.

4. CONCLUSION (1) The composition of raw materials has played a critical role on mesophase pitch formation. The mesophase pitch with larger domains optical texture was produced due to higher aromaticity, shorter alkyl side chain and more naphthenic groups in the raw materials. Additionally, the anisotropic fraction and the soluble mesophase contents increased with the softening points decreased gradually. During the process of thermal


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treatments, the intermediate components HI-TS and TI-PS yields of P1-MP, P2-MP and P3-MP changed slowly and the thermal polycondensation reaction was restrained. (2) With the increase of naphthenic groups and shortening of alkyl side chain length in raw materials, the PAH of P1-MP, P2-MP and P3-MP decreased respectively, indicating that the alkyl groups contents of the three kind of mesophase pitches increased. It was consistent to the change of aromatic indexes. The mesophase pitch became highly crystalized as shown in XRD spectra. As the result, the stack height (Lc) increased, the interlayer spacing (d002) decrease and the alignment degree (Og) was improved. (3) The raw oil with higher aromaticity, more naphthenic groups and a certain amount of short alkyl side chains is benefit to obtain the mesophase pitch with lower softening point, better optical and crystal structure.

AUTHOR INFORMATION Corresponding Author * Tel. /tax: +86 0532-86984629. E-mail address: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21176259) and the Fundamental Research Funds for the Central Universities (15CX05009A).


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Preparation of the Mesophase Pitch by Hydrocracking Tail Oil from a

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