Relationship between Structural Modification of Aromatic-Rich

Article pubs.acs.org/EF

Relationship between Structural Modification of Aromatic-Rich Fraction from Heavy Oil and the Development of Mesophase Microstructure in Thermal Polymerization Process Ming Li,† Dong Liu,*,† Bin Lou,† Xulian Hou,‡ and Peng Chen† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, China, 266580 China Petroleum Engineering Co., Ltd., Beijing, China, 100085

‡

ABSTRACT: In this study, the aromatic-rich fraction from heavy oil was modified by polyacrylic acid and oleic acid to obtain the modified raw materials which were used for producing mesophase pitch via polymerization reaction. First, the properties of modified feedstocks were analyzed by Fourier transform infrared spectrometry (FTIR), simulated distillation, and elemental analysis. The optical texture and molecular and microcrystal structure of the mesophase were characterized by the techniques of polarized optical microscopy, FTIR, 1H nuclear magnetic resonance, X-ray diffraction, and Raman spectroscopy. The influence of alkyl structures contained in modified feedstocks on the preparation of the mesophase was studied. The results suggested that the amounts of alkyl chains in modified raw materials increased first and then decreased with raising the amount of the additives. The short and long alkyl chains in mesophase pitches from modified feedstocks were resulted by treatment of polyacrylic acid and oleic acid, respectively. In addition, the mesophase pitches with finer microcrystal structures, less crystalline imperfection, and high degree of graphitization were generated from modified raw materials. Moreover, an appropriate amount of short alkyl chains or a small number of long alkyl chains was favorable for the formation of a mesophase with a large domain structure, low softening point, high carbon residue, and fine microcrystal structure.

1. INTRODUCTION Mesophase pitch (MP) is commonly acknowledged as an excellent intermediate for preparing high quality carbon materials.1 Mesophase pitch based carbon material has great exploitation potentiality and broad application prospects in the field of new carbon materials due to its low price, high strength, good electric performance, environmental protection, etc.1−4 Heavy oil which contains a high content of aromatic fractions can be used for the preparation of high-quality mesophase pitch; moreover, the vacuum residue is abundant in source and cheap in price.5−9 Therefore, the preparation and formation mechanism of petroleum based mesophase pitch are the research hotspots worldwide in the field of carbon materials. Many researches around the properties of raw material emphasized the significance of alkyl structures on the generation of mesophase pitch.10−12 Previously, studies showed that the qualities of a mesophase strongly depended on the number and length of alkyl chains contained in the feedstock.10−12 Some researchers demonstrated the influence of the alkyl chain’s length on the development of a mesophase through the alkylation method.11−14 Miyake et al. have treated a kind of mesophase pitch containing few alkyl groups with different additives through alkylation reaction to get alkylated mesophase pitch (contains methyl and ethyl).15 This study found that the content of the anisotropic structure varied evidently with the change of the number and steric size of alkyl structures. In addition, Mochida et al. studied the development of a mesophase prepared by naphthalene and methyl naphthalene, respectively. They suggested that the quinoloneinsoluble fractions of pitches were converted into graphitizable carbon effectively through the alkylation methods.16−19 Furthermore, Korai et al.20 investigated the role of methyl © XXXX American Chemical Society

(−CH3) on the structure of mesophase molecules. They considered the methyl substituents as the critical factor for affecting the ordering of mesophase molecules. Moreover, the influence of methyl substituents contained in raw material on the preparation of mesophase pitch has also been investigated by Yoon et al.21 In conclusion, it is worth to study the role alkyl structures have played on the development of mesophase pitch comprehensively. In this paper, the aromatic-rich fraction derived from heavy oil was subjected to alkylation treatment by reacting with polyacrylic acid or oleic acid, and then those modified feedstocks were treated by direct thermal polymerization to investigate the relationship between structural modification of raw materials and development of a mesophase microstructure during the thermal polymerization process. Moreover, this paper investigated the effect of the number and length of alkyl chains on the qualities of MP and discussed the mechanism of carbonation.

2. EXPERIMENTAL SECTION 2.1. Materials. The aromatic hydrocarbons in heavy oil were from Shengli Refinery of Qilu Petrochemical Industries Co., Sinopec. The polyacrylic acid and oleic acid were supplied by the Sinopharm Chemical Reagent Co., Ltd. Company. The molecular weight of polyacrylic acid is in the range of 800−1000. 2.2. Chemical Modification and Thermal Polymerization. The aromatics enriched fraction together with additives (polyacrylic acid or oleic acid) was heated in the 300 mL high-pressure autoclave Received: June 19, 2016 Revised: August 9, 2016

A

DOI: 10.1021/acs.energyfuels.6b01496 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels under the temperature of 380 °C after replacing air in the reaction system by nitrogen. When the reaction was carried out for 2 h under autogenic pressure, the reaction system was purged by nitrogen for 30 min to remove the additives. Finally, the modified raw materials were obtained. The modified feedstock was put into the agitated autoclave (300 mL) and treated at 440 °C under 4 MPa for 6 h to prepare mesophase pitch. 2.3. Characterization of Samples. The elemental composition of the modified material was determined by the Vario EL II element analyzer made by the Chnos Company. FTIR spectra of samples were recorded on an FTIR spectrometer (Nicolet S-215). The modified materials’ aromaticity (fa) and molar ratios of −CH2− to −CH3 (r) were calculated according to the following formulas.22 The parameter P is calculated by the formula: A1600/(A1600 + 0.16A1460 + 0.23A1330). The absorption peak intensities at 1330, 1380, 1460, and 1660 cm−1 were denoted as A1330, A1380, A1460, and A1660, respectively. The characteristic peaks at 1460 and 1380 cm−1 represent the groups of −CH2− and −CH3. fa = 0.574P + 0.024

(1)

A1460 − 3.72 A1380

(2)

r = 3.07

distillation are shown in Figure 1. The initial boiling points (IBP) and final boiling points (FBP) of F, M4, and N4, as well

The optical texture of the product was analyzed by a polarized microscope (XP-4) made by the Milite Company, China. The dividing standard of the types of optical textures is summarized in Table 1.23

Table 1. Classification of Mesophase Microstructures microstructures type

feature size/μm

mosaic structure small domain structure medium domain structure large domain structure

200

Figure 1. Distillation characteristics of M4 and N4 via simulated distillation.

The carbon residues of samples were measured according to ASTM D4530. Three solventsn-heptane, toluene, and pyridinewere used to analyze the solubility of mesophase pitch.24 The mesophase pitch was separated into four fractions: n-heptane-soluble fraction (HS), nheptane-insoluble/toluene-soluble fraction (HI-TS), and tolueneinsoluble fraction (TI). The 1H nuclear magnetic resonance (NMR) spectra of products were recorded on a Unity-200 MHz FT NMR spectrometer made by the US Varian Company using deuterated pyridine as the solvent. The X-ray diffraction of mesophase pitches was measured by a PANalyitcal X Pert PRO MPD using Cu Kα radiation at a wavelength of 0.15418 nm. A Labram 10 Raman spectrometer made by the Jobin Yvon Company was used to determinate the Raman spectra of mesophase pitches. The monochromatic red light (λ = 632.8 nm) was gained by a He−Ne laser at 1 mW.

Table 2. IBP and FBP of Modified Materials, Boiling Points of Additives samples

IBP/°C

FBP/°C

boiling point of additives/°C

M4 N4

337.5 380.9

622.7 633.3

116 350−360

as the boiling points of additives, are listed in Table 2. The IBP of M4 was below the boiling point (116 °C) of its corresponding additive (polyacrylic acid), and the IBP of N4 was below the boiling point (350−360 °C) of oleic acid. This indicated that the modified materials contain no additives after alkylation reaction. The elemental components of modified materials are listed in Table 3. Compared with the F, the n(H)/n(C) atomic ratio of modified materials all increased. In addition, the oxygen mass percents of modified materials (M1, M2, M3, M4, N1, N2, N3, and N4) were all below 0.02‰, which indicated that there was almost no oxygen-containing functional group in the modified materials. Therefore, the influence of oxygen was ignored in the following carbonation reaction. 3.1.2. FTIR Analysis of Modified Materials. Figure 2 show the FTIR spectra of modified materials F, M1, M2, M3, M4, N1, N2, N3, and N4. The modified materials’ aromaticity ( fa) and molar ratios of −CH2− to −CH3 (r) are listed in Table 4. The absorption peaks of polycyclic aromatic hydrocarbons between the region of 700−900 cm−1, the band of −CH2−

3. RESULTS AND DISCUSSION 3.1. Analysis of Modified Materials. 3.1.1. Simulated Distillation and Elemental Analysis of Modified Materials. The raw materials modified with 5, 10, 20, and 30 wt % of polyacrylic acid were labeled as M1, M2, M3, and M4, respectively. Similarly, N1, N2, N3, and N4 referred to the samples obtained from modification with 5, 10, 20, and 30 wt % of oleic acid. In addition, the raw material treated without additives under the same conditions was named F. In order to illustrate whether the modified materials contain additives after the modification reaction, the modified materials were analyzed by simulated distillation. If the modified materials with the largest amount (30 wt %) of additives contain no additives, we considered that there were no additives left in the modified materials after alkylation reaction. The distillation characteristics of M4 and N4 via simulated B


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N4 decreased in a similar way. It implied that the contents of alkyl structures in modified raw materials increased with the increase of additive amounts. Moreover, the aromatic degrees ( fa) of modified raw materials were lower than F, also indicating that the modified raw materials contained more alkyl structures than F. In addition, the molar ratios (r) of −CH2− to −CH3 of modified raw materials (M1, M2, M3, M4, N1, N2, N3, and N4) were all larger than the ratio of F, suggesting that the alkyl chain length of modified raw materials were all longer than the length of F. It meant that the modified raw materials contained long alkyl chains after alkylation reaction. On the other hand, the ratios (r) of M1, M2, M3, and M4 had no obvious change, which implied that the modified feedstocks were similar in the length of the alkyl chain. Analogously, the change of the ratios (r) of N1, N2, N3, and N4 showed the same trend, indicating that the alkyl chain length of N1, N2, N3, and N4 varied very little. However, the ratios (r) of N1, N2, N3, and N4 were all larger than those of M1, M2, M3, and M4; it meant that the lengths of alkyl chains in N1, N2, N3, and N4 were longer than those in M1, M2, M3, and M4. To sum up, the contents of alkyl chains increased with the addition of additive amounts in modified raw materials. Meanwhile, the alkyl chains contained in modified feedstocks treated by polyacrylic acid were shorter than the chains in modified raw materials modified by oleic acid. Therefore, the modified feedstocks treated by polyacrylic acid were recognized to contain short alkyl chains, whereas the modified raw materials modified by oleic acid were deemed to have long alkyl chains. During the carbonization process, the polyacrylic acid and oleic acid were decomposed into hydroxide radicals, carbon dioxide, and alkyl radicals in the primary reaction.26−28 Meanwhile, the aromatic molecule radicals, hydrogen radical, and other free radicals with small molecules were generated by the pyrolysis of raw material.29,30 When the temperature and pressure reached a certain level, the alkyl radicals could react with the aromatic molecule radicals to form aromatic macromolecules with different lengths of alkyl chains.31,32 The above FTIR analysis illustrated that the short and long alkyl chains in modified feedstocks were produced by treatment of polyacrylic acid and oleic acid, respectively. 3.2. Optical Microstructure of Mesophase Pitch. Figure 3 shows the optical micrographs of mesophase pitches prepared by F, M1, M2, M3, M4, N1, N2, N3, and N4 (named F-MP, M1-MP, M2-MP, M3-MP, M4-MP, N1-MP, N2-MP, N3-MP, and N4-MP, respectively). From Figure 3(1), it was observed that the medium domain structure with a size in the range of 100−200 μm appeared in the optical micrograph of F-MP. The anisotropic structure developed with the increase of polyacrylic acid used for modification of feedstock. When the additive amount of polyacrylic acid was 20 wt %, the large domain optical structure with a diameter larger than 200 μm was formed (shown in the optical micrograph of M3-MP in Figure 3(1)). However, with the growing amount of polyacrylic acid (30 wt %), the mosaic

Table 3. Elemental Components of Modified Materials sample

H/wt %

C/wt %

O/wt %

n(H)/n(C)

F M1 M2 M3 M4 N1 N2 N3 N4

8.37 8.59 8.46 8.53 8.39 8.40 8.58 8.64 8.41

89.84 89.83 89.99 89.77 90.01 90.13 90.18 90.05 90.17

0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

1.1180 1.1475 1.1281 1.1402 1.1185 1.1184 1.1417 1.1514 1.1192

Figure 2. FTIR spectra of modified feedstocks: (1) F, M1, M2, M3, and M4; (2) F, N1, N2, N3, and N4.

(near 1460, 2853, and 2923 cm−1), and the absorption band of −CH3 (at 1380 and 2960 cm−1) could be observed in FTIR spectra of modified feedstocks (shown in Figure 2). It demonstrated that the modified materials were polycyclic aromatic hydrocarbons with many alkyl chains.25 Table 4 shows that the aromatic degree (fa) of M1, M2, M3, and M4 decreased successively, and those of N1, N2, N3, and

Table 4. Aromaticity (fa) and Molar Ratios of −CH2− to −CH3 (r) of Modified Feedstocks sample fa r

F

M1

M2

M3

M4

N1

N2

N3

N4

0.36 1.75

0.31 1.96

0.27 1.95

0.25 1.93

0.24 1.97

0.32 2.11

0.29 2.14

0.28 2.09

0.27 2.22

C


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Figure 3. Optical micrographs of mesophase pitches: (1) F-MP, M1-MP, M2-MP, M3-MP, and M4-MP; (2) F-MP, N1-MP, N2-MP, N3-MP, and N4-MP.

structure with a diameter smaller than 50 μm occurred in M4MP. This behavior could be explained by the overabundance of short alkyl chains that would generate many free radicals at high temperature, which could accelerate the polymerization of macromolecular radicals and prevent the formation of mesophase pitch with an ordered molecular structure.33,34 In Figure 3(2), a large domain optical texture appeared (shown in the optical micrograph of N3-MP) when the additive amount of oleic acid in the raw material was 5 wt %. However, with the increase of the additive amount (10, 20, and 30 wt %), the optical structure size was getting smaller. This phenomenon was due to the excessive long alkyl chains in the modified materials which could accelerate the reaction rate and was inconducive to the development of mesophase pitch.33,34 During the thermal cracking process, the active radicals were produced through dealkylation reaction. The reaction activation energy was reduced due to the presence of alkyl chains. Therefore, the rate of condensation reaction was promoted, which was favorable for the development of an anisotropic structure.32−35 As a consequence, the mesophase pitch with a domain structure was generated easily. However, if the modified materials contained an excess of short alkyl chains

(when the additive amount of polyacrylic acid was 30 wt %) and long alkyl chains (when the additive amount of oleic acid was 10, 20, and 30 wt %), more free radicals would be generated, which could improve the reactivity and accelerate the rate of condensation polymerization. Therefore, the viscosity of the system increased rapidly and there was not enough time for the aromatic molecule layers to be arranged orderly. As a result, the molecular structure of the mesophase pitch became worse, which led to the mosaic texture with a small diameter. Therefore, an appropriate amount of short alkyl chains or a small quantity of long alkyl chains that existed in the feedstocks was favorable for the preparation of mesophase pitch with domains texture. 3.3. Softening Points and Carbon Residue of Mesophase Pitch. The softening points and carbon residue of mesophase pitch (F-MP, M1-MP, M2-MP, M3-MP, M4-MP, N1-MP, N2-MP, N3-MP, and N4-MP) are listed in Table 5. With the increase of the additive amount of polyacrylic acid and oleic acid, the softening points of mesophase pitches decrease first and then increased. The change tendency of the carbon residues was opposite to that of softening points. This behavior could be explained by the amount of alkyl radicals D


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Energy & Fuels Table 5. Softening Points and Carbon Residue of Mesophase Pitch sample

softening point/°C

carbon residue/%

F-MP M1-MP M2-MP M3-MP M4-MP N1-MP N2-MP N3-MP N4-MP

259 242 234 229 241 232 237 246 268

73.68 74.85 79.26 85.40 80.97 81.29 76.81 74.90 73.36

produced by the modified materials with alkyl chains during the pyrolytic process. The alkyl side chains were easily broken when the temperature was high enough and the free radicals with small molecule were produced. The small molecular substances could play a role as solvent, which could decrease the viscosity of the system. It would avoid the aggregation of macromolecular substances caused by the overreaction effectively. Therefore, in a certain range of contents, the softening points decrease with the increase of alkyl side chains. Meanwhile, the free radicals with a small molecule easily formed large molecular substances at low viscosity, which could reduce the overflow of light components. Therefore, the carbon residues increased first. However, when the modified materials contained too many alkyl chains, the fast reaction rate led to the excessive condensation, and then the softening points increased and the carbon residues decreased. 3.4. Solubility Analysis of Mesophase Pitch. The variations of the HS, HI-TS, and TI yields with the growth amount of polyacrylic acid and oleic acid are shown in Figure 4. Figure 4(1) shows that the yields of HS and HI-TS fractions decreased with raising the amount of polyacrylic acid, whereas the TI fractions increased. The change tendencies of the three fractions with the adding amount of oleic acid were consistent with those of polyacrylic acid (shown in Figure 4(2)). The reason was that the free radicals produced by modified materials with alkyl chains could reduce the activity energy efficiently and speed up the rate of reaction distinctly. Therefore, the reaction rate became faster with the growth of free radicals in the reaction system. In addition, the existence of alkyl chains in modified materials could increase the solubility of the reaction system, which was beneficial to the formation of macromolecular substances. Therefore, the contents of compositions containing small molecules (HS and HI-TS) in mesophase pitches decreased, while the macromolecular substances (TI) increased with raising the amounts of additives. 3.5. FTIR Analysis of Mesophase Pitch. The FTIR spectra of mesophase pitches (F-MP, M1-MP, M2-MP, M3MP, M4-MP, N1-MP, N2-MP, N3-MP, and N4-MP) can be seen in Figure 5. The characteristic absorption peaks of alkyl C−H, aromatic C−H, and methyl −CH3 at 2920, 3040, and 1380 cm−1, respectively, could be seen in the spectra of modified feedstocks (shown in Figure 5). This implied that the mesophase pitches with a certain number of alkyl chains were generated from the modified materials.35 The aromatic indexes (Iar) of mesophase pitches were calculated according to formula 3,22 and the results are listed in Table 8. The data suggested that the contents of alkyl structures contained in the mesophase

Figure 4. Change of HS, HI-TS, and TI contents with the increase of polyacrylic acid and oleic acid.

increased with the growth of polyacrylic acid and oleic acid, and they were all higher than the alkyl structures contained in F-MP. Iar =

Ab3040 Ab3040 + Ab2920

(3)

The condensation degrees of mesophase pitches were characterized by the peak intensity ratio of aromatic C−H (at 800 cm−1) to aromatic CC (at 1600 cm−1) (formulated as Ab880/Ab1600),22 and the calculated results are expressed in Table 6. The data demonstrated that the condensation degrees of mesophase pitches decreased with raising the amount of polyacrylic acid and oleic acid, and they were all lower than that of F-MP. 3.6. 1H NMR Analysis of Mesophase Pitch. Figure 6 expresses the 1H NMR spectra of the pyridine-soluble fractions (PS) of F-MP, M3-MP, and N1-MP (named F-MP-PS, M3MP-PS, and N1-MP-PS, respectively), and Table 7 lists the hydrogen distribution. As shown in Table 7, compared with F-MP-PS, the contents of Har in M3-MP-PS and N1-MP-PS decreased, whereas the contents of Hβ, Hγ, and Hn increased, which indicated that M3MP-PS and N1-MP-PS contained more alkyl structures than FMP-PS. The contents of Hn in N1-MP-PS and M3-MP-PS were higher than that in F-MP-PS, demonstrating that N1-MP-PS and M3-MP-PS possessed more naphthenic structures than FMP-PS. Additionally, compared with M3-MP-PS, N1-MP-PS possessed less Hβ but more Hγ. It implied that the length of alkyl chains in mesophase pitch M3-MP-PS was shorter than E


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3.7. XRD Analysis of Mesophase Pitch. The X-ray diffraction (XRD) spectra of the three kinds of mesophase pitches can be seen in Figure 7. The strong diffraction peaks were exhibited in all the spectra of F-MP, M3-MP, and N1-MP, demonstrating that the mesophase pitches were all wellcrystallized. The crystal parameters of the mesophase pitches are listed in Table 8. Compared with F-MP, the stack heights (Lc) of the M3-MP and N1-MP were higher, the interlayer spacings (d002) were smaller, the layer number (n) became larger, and the alignment degrees (Og) of the mesophase pitches were improved. It implied that the mesophase pitches from modified raw materials became highly crystallized. That is to say, the crystal structure of mesophase pitch was improved when the raw materials were alkylated by polyacrylic acid and oleic acid. In addition, compared with N1-MP, the mesophase pitch M3-MP showed better crystal structure: higher stack height, smaller interlayer spacing, more layers, and improved alignment degrees. It revealed that the alkyl chains contained in modified materials had an enhanced effect on the crystal structure of mesophase pitch, and the effect of short alkyl chains was better than that of long alkyl chains. 3.8. Raman Spectroscopy Analysis of Mesophase Pitch. The Raman spectra of F-MP, M3-MP, and N1-MP can be observed in Figure 8. The G peak position (Pos(G)) and its full width at half-maximum (fwhm(G)) are obtained from the center position and the standard deviation (SD) of the Gaussian distribution function simulating the G peak.39,40 The Raman parameters are listed Table 9 As shown in Table 9, the disordered carbon or defect mode D peak (at 1360 cm−1) and the typical graphite lattice vibration mode G peak (at 1580 cm−1) were all displayed in the spectra of F-MP, M3-MP, and N1-MP. Compared with F-MP, the ID/ IG ratios of M3-MP and N1-MP were lower, implying that their order degrees increased after alkylation modification. This phenomenon could be explained by the change of microcrystal structure caused by modification of polyacrylic acid and oleic acid. Meanwhile, the half-peak breadth (D) of M3-MP and N1MP became narrow and the grain size (La) increased, demonstrating that the crystalline imperfection decreased and the degree of graphitization improved. In conclusion, the mesophase pitches generated by modified raw materials had less crystalline imperfection and showed a high degree of graphitization. Moreover, the crystal structure of M3-MP was more orderly than N1-MP, because the crystalline imperfection of M3-MP was less and the degree of graphitization was higher. In a word, an appropriate amount of short alkyl chains in the modified material was helpful to the formation of high quality mesophase pitch with low softening point, high carbon residue, and fine optical and crystal structure.

Figure 5. FTIR spectra of MP: (1) F-MP, M1-MP, M2-MP, M3-MP, and M4-MP; (2) F-MP, N1-MP, N2-MP, N3-MP, and N4-MP.

that in N1-MP-PS.12,36 The increased amount of naphthenic structures and alkyl chains decreased the steric hindrance of the aromatic intermediates, which was beneficial for the orderly stacking of aromatic layers.37 The aliphatic hydrogens in mesophase pitches were mainly at α- and β-positions, demonstrating that a number of methyl (CH3−) and methylene (−CH2−) substituents existed in mesophase pitches. The absorption peaks of −CH2− in the range of 3.3−4.2 could be observed in Figure 6, suggesting that methylene substituents played a role as a bridge to connect aromatic molecules with each other.37,38 During the developing of mesophase pitch, the low condensation degree of aromatic macromolecules was favorable to the generation of mesophase pitch with an ordered molecular structure.11,12,38

4. CONCLUSION (1) The preparation of mesophase pitch has strongly depended on the composition of raw materials. The contents of alkyl structures in mesophase pitches from

Table 6. Aromatic Indexes (Iar) and Ratios of Ab880/Ab1600 of Mesophase Pitches sample Iar Ab880/Ab1600

F-MP

M1-MP

M2-MP

M3-MP

M4-MP

N1-MP

N2-MP

N3-MP

N4-MP

0.851 1.548

0.834 1.521

0.812 1.367

0.735 1.034

0.749 1.247

0.795 1.492

0.802 1.501

0.836 1.542

0. 857 1.558

F


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Figure 6. 1H NMR spectra of F-MP-PS, M3-MP-PS, and N1-MP-PS.

Table 7. Hydrogen Distribution of the Pyridine-Soluble Fractions of the Mesophasea sample

Har/%

Hα/%

Hβ/%

Hγ/%

Hn/%

F-MP-PS M3-MP-PS N1-MP-PS

79.8495 72.3082 75.5695

18.0047 20.3509 13.4769

0.7735 2.9408 6.0672

0.9175 4.0721 3.6294

0.4548 2.3688 1.2571

Table 9. Raman Parameters of F-MP, M3-MP, and N1-MP

Har: δ = 9.0−6.0 ppm, aromatic hydrogens; Hα: δ = 4.0−2.0 ppm, aliphatic hydrogens in α-position to the aromatic carbon; Hβ: δ = 1.4− 1.0 ppm, aliphatic hydrogens in β-position to the aromatic carbon; Hγ: δ = 4.0−2.0 ppm, aliphatic hydrogens in γ-position to the aromatic carbon; Hn: δ = 2.0−1.4 ppm; naphthenic hydrogen.22,37

sample

ID/IG

D/cm−1

La (×10−2)

F-MP M3-MP N1-MP

0.456099 0.415548 0.428072

55 49 51

1.205879 1.323553 1.284831

a

(2)

(3) (4)

(5)

■

Figure 7. XRD spectra of F-MP, M3-MP, and N1-MP.

Table 8. Crystal Parameters of F-MP, M3-MP, and N1-MP code

d002/Å

Lc/nm

n

Og

F-MP M3-MP N1-MP

3.47 3.42 3.45

2.75 3.01 2.90

7.96 9.97 8.39

0.9150 0.9571 0.9230

modified feedstocks treated with polyacrylic acid and oleic acid increased first and then decreased with raising the amounts of additives. The contents of compositions containing small molecules (HS and HI-TS) in mesophase pitches from modified feedstocks decreased, whereas the macromolecular substances (TI) increased with raising the amounts of additives. The short and long alkyl chains in mesophase pitches from modified feedstocks were resulted by treatment of polyacrylic acid and oleic acid, respectively. The mesophase pitches from modified raw materials with a more orderly microcrystal structure, less crystalline imperfection, and high degree of graphitization were generated through carbonation. An appropriate amount of short alkyl chains or a small number of long alkyl chains was favorable for the preparation of mesophase pitch with a large domain structure, high carbon residue, low softening point, and fine microcrystal structure.

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 0532-86980381. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work has been financially supported by the National Natural Science Foundation of China (21176259). REFERENCES

(1) Tate, K.; Yoshida, H.; Yanagida, K. Pitch for production of carbon fibers. U.S. Patent 4,670,129, 1987. (2) Fathollahi, B.; Chau, P. C.; White, J. L. Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites. Part I. Injection process. Carbon 2005, 43 (1), 125−133. (3) Fathollahi, B.; Chau, P. C.; White, J. L. Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites: part II. Stabilization process. Carbon 2005, 43 (1), 135−141. (4) Fathollahi, B.; Jones, B.; Chau, P. C.; White, J. L. Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites. Part III: mesophase stabilization at low temperatures and elevated oxidation pressures. Carbon 2005, 43 (1), 143−151. (5) Santamaría-Ramírez, R. R.; Romero-Palazón, E.; Gómez-deSalazar, C.; Rodríguez-Reinoso, F.; Martínez-Saez, S.; MartínezEscandell, M.; Marsh, H. Influence of pressure variations on the

Figure 8. Raman spectra of F-MP, M3-MP, and N1-MP.

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Relationship between Structural Modification of Aromatic-Rich

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