Effect of carbonaceous precursors on the structure of mesophase

Article Cite This: Energy Fuels 2018, 32, 8329−8339

pubs.acs.org/EF

Effect of Carbonaceous Precursors on the Structure of Mesophase Pitches and Their Derived Cokes Guanming Yuan,*,† Zhao Jin,† Xiaohua Zuo,‡ Zheng Xue,† Feng Yan,† Zhijun Dong,† Ye Cong,† and Xuanke Li*,† †

The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, China, 430081 School of Chemistry & Chemical Engineering, Hubei Polytechnic University, Huangshi, China, 435003

Energy Fuels 2018.32:8329-8339. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/17/18. For personal use only.

‡

ABSTRACT: Using six representative feedstocks as carbonaceous precursors, various mesophase pitches and their derived cokes were obtained by a heat-soaking method at a temperature range of 400−450 °C and a pyrolysis treatment at 900 °C, respectively. The optical texture and microstructure of the different mesophase pitches and their derived cokes were characterized by polarized-light microscope, scanning electron microscope, and X-ray diffraction. The results show that the formation and development abilities of liquid crystalline mesophase are obviously different for the various feedstocks. C9 aromatic hydrocarbons and petroleum-derived paving pitch possess relatively high thermo-chemical reactivity due to the existence of low molecular substances and aliphatic groups, leading to form liquid crystalline mesophase easier and faster than those of naphthalene synthetic pitch and coal tar-based impregnating pitch under a similar condition, which however is undesirable to develop a bulk mesophase. Anthracene possesses a characteristic of forming a homogeneous bulk mesophase with a streamline texture under a suitable condition. The liquid crystalline transformation behavior of C5−C9 aromatic hydrocarbons is unexpectedly similar to that of naphthalene synthetic pitch and markedly different from that of C9 aromatic hydrocarbons. Carbonaceous precursors have a significant effect on the anisotropic content and the optical texture of the mesophase pitch products, which leads to the microstructure variety of the resultant cokes. Fine-grained and coarse-grained mosaic textures are, respectively, present in the cokes derived from C9 aromatic hydrocarbons and petroleum-derived paving pitch. The cokes derived both from coal tar-based impregnating pitch and from C5−C9 aromatic hydrocarbons possess a supra mosaic texture mingled with a local flow-induced orientation domain. A well-oriented lamellar texture is clearly found in the cokes derived from anthracene and naphthalene synthetic pitch.

1. INTRODUCTION Mesophase pitch with a multiple-discoid shape is a complex mixture of polycyclic aromatic hydrocarbons with a relative molecular weight of 370−2000.1−4 Its research can be traced back to the 1960s, when Books and Taylor found there was liquid crystalline phase (i.e., mesophase pitch) in the liquidphase carbonization of pitch. The optical texture, chemical structure, and formation mechanism of mesophase pitch were investigated.5 Subsequently, the research and application of mesophase pitches prepared either through thermal polycondensation of coal tar pitch, heavy oil, coal tar, and ethylene tar, etc., or by catalytic polymerization of aromatic compounds have been developing rapidly.6−9 The mesophase pitch with a nematic liquid crystal structure can be preferentially aligned under mechanical force shearing after melting; thereby, it is regarded as a basic raw material for preparing high-performance carbon materials with controllable structure of forming an ordered graphite, which provides a feasible route to prepare graphite-like materials (i.e., possessing an easily graphitizable characteristic).6,7 Due to the high carbon yield and potential price advantage (owing to the low cost of raw materials) of mesophase pitch, it has become a high-quality precursor material for high-performance and multifunctional carbon materials. Mesophase pitch is widely used in mesophase pitchbased coke, needle coke, high-power graphite electrodes, mesocarbon microbeads, carbon foam, mesophase pitch-based carbon fibers, and binder and impregnating agent for high© 2018 American Chemical Society

thermal-conductivity carbon/carbon composites. Therefore, mesophase pitch occupies a pivotal and irreplaceable position in various fields, such as defense, military, aerospace, cuttingedge technology, high-end industrial manufacturing, etc.7,10 Mesophase pitch-based carbon fibers are the most successful high-end product for the development and application of mesophase pitch, which are derived from spinnable mesophase pitch by hot-melt spinning, oxidative stabilization, and hightemperature carbonization/graphitization treatments. The inherent alignment structure of liquid crystal molecules is preserved within the as-spun pitch fibers. Upon hightemperature graphitization, the graphite crystals are preferentially oriented along the fiber axis, so the final fibers have super high Young’s modulus, excellent axial electrical and thermal conductivity.11−14 Mesophase pitch-based carbon fibers can be widely used in aviation, aerospace, nuclear and other high-tech fields, in which polyacrylonitrile-based carbon fibers have a certain limitation.10,15 It is well-known that the carbonaceous raw materials are very important for the synthesis of high-quality mesophase pitch, which will undoubtedly dominate the microstructure and physical properties of resulting carbon materials. The structure within pitch-derived coke (i.e., needle coke, a graphitizable Received: May 25, 2018 Revised: July 24, 2018 Published: July 24, 2018 8329

DOI: 10.1021/acs.energyfuels.8b01824 Energy Fuels 2018, 32, 8329−8339

Article

Energy & Fuels Table 1. Physical Properties of Various Carbonaceous Precursors main fraction/wt %a feedstock

HS

HI-TS

anthracene naphthalene synthetic pitch coal tar-based impregnating pitch petroleum-derived paving pitch C9 aromatic hydrocarbons C5−C9 aromatic hydrocarbons

95.50 17.28 16.46 75.98 29.20 100.00

4.50 76.47 58.90 24.02 70.80

TI-QS

QI

6.25 24.64

softening point/°C

coking value/wt %b

ash content/ppm

78 107 78 52 93 91

1.10 54.00 59.70 8.45 10.12 6.20

490 500 300 770 200 430

a

Main fraction (i.e., hexahydrobenzene soluble, hexahydrobenzene insoluble and toluene soluble, toluene insoluble and quinoline soluble, quinoline insoluble, marked with HS, HI-TS, TI-QS, and QI in sequence) was measured by solvent extraction treatments using hexahydrobenzene, toluene, and quinolone. bCoking value was determined by 900 °C carbonization treatment for 1 h.

2. EXPERIMENTAL SECTION

carbon) is governed by the properties of the mesophase pitch at the time of its solidification and these are closely dependent on the parent feedstocks. For example, raw materials for the fabrication of spinnable mesophase pitch used for carbon fibers are critical. Although ordinary coal tar pitch or petroleum pitch are cheap and available, the pitch raw materials are a complex mixture and their quality is unstable and comparatively difficult to control owing to the variety of production technology. The components in the raw materials are extremely complicated, and the content of heteroatoms and ash is relatively high. Even the ash content of impregnating pitch for carbon materials is also higher than 200 ppm. There are not plenty of suitable components in pitch for the preparation of high-quality mesophase pitch, so it is necessary to carry out complex purification, “cutting” certain components, refining and modification treatments, i.e., filtration, distillation, solvent extraction, hydrogen reduction, cocarbonization, catalytic reforming, etc. Thus, the preparation processing undoubtedly greatly increases the production cost and technical complexity.16,17 In recent years, pure aromatic hydrocarbon feedstocks (i.e., naphthalene, methylnaphthalene, and anthracene, etc.) are selected as carbonaceous precursors to catalytically synthesize spinnable mesophase pitch. This route can not only avoid the complicated refining process of pitch and reduce the production cost but also facilitate the synthetic pitch products with high purity, narrow molecular weight distribution, and uniformly regular structure, which is more suitable for preparing high-performance carbon fibers.10,18,19 Thus, it can be seen that the carbonaceous precursors, which dominate the boom and fate of some advanced carbon materials, are extraordinarily important for the preparation of high-quality mesophase pitch. In this work, various raw materials including a pure aromatic compound and aromatic hydrocarbon mixture are selected as carbonaceous precursors to prepare mesophase pitches. The optical texture, main components, and physical properties of mesophase pitch products made at different heat-soaking temperatures are systematically studied. The thermal-chemical reaction activity and the formation and transformation abilities of liquid crystalline mesophase for the different feedstocks are investigated. The influences of carbonaceous precursors on the size and the shape of optical texture of resulting mesophase pitch products are discussed in detail. In addition, the optical texture, microstructure, and crystal orientation of mesophase pitch-derived cokes are also characterized in order to clarify which precursor is more suitable for preparing high-quality mesophase pitch.

2.1. Materials. Six representative feedstocks (i.e., anthracene, naphthalene synthetic pitch, coal tar-based impregnating pitch, petroleum-derived paving pitch, C9 aromatic hydrocarbons, and C5−C9 aromatic hydrocarbons) with a relatively high purity (≥99.9%) were used as carbonaceous precursors to prepare mesophase pitches and their derived cokes. Naphthalene synthetic pitch was synthesized in the laboratory by catalytically polymerizing pure naphthalene (naphthalene was not directly used as a raw material owing to its strong volatilization and chemical stability, which may result in an ultralow thermal polymerization yield without a catalyst) using anhydrous AlCl3 and then removing the catalyst as less as possible. The other feedstocks were commercially purchased. 2.2. Preparation of Mesophase Pitches and Cokes. About 50 g of feedstock was placed in a 50 mL ceramic pot with lid and then heat-treated under a nitrogen atmosphere in a crucible furnace using a heating rate of 2 °C/min, the heat-soaking temperature was selected as 420 °C, and the soaking time was fixed at 4 h. For the feedstocks possessing relatively high or low chemical reaction activity, the soaking temperatures of 410 °C (or even 400 °C) and 430 °C (or even 440 and 450 °C) were also performed for a comparison. The as-prepared mesophase pitch products were divided into two parts. One part was used for analysis and characterization, and the other was subsequently carbonized at 900 °C for 1 h under a nitrogen atmosphere in a crucible furnace using a heating rate of 1 °C/min in order to obtain the cokes. 2.3. Characterization of Mesophase Pitches and Cokes. The solubility of carbonaceous feedstocks and their mesophase pitch products was analyzed by dividing the test sample into four or three components via sequential extraction treatments using hexahydrobenzene, toluene, and quinolone as solvents. The chemical structure changes and the aromatic indices of carbonaceous precursors and their derived mesophase pitch products were analyzed by a Bruker Vertex 70 Fourier transform infrared spectrometry (FTIR). Mesophase pitches and their derivative cokes were embedded in a block of polyester resin and then carefully polished with a fine sand paper. The surfaces of the pitches and cokes were then observed under a Carl Zeiss AX10 polarized light microscope (PLM) in reflectance mode so as to obtain the optical texture and optically anisotropic content of liquid crystalline mesophase,20 the magnification of each polarized photomicrograph was 100×, and the scale bar was 100 μm (Figure 3e was exceptionally magnified 200 times in order to get the clear image). The softening points of various mesophase pitch products were measured on a hot-stage with flowing nitrogen under the PLM. The morphology and microstructure of different cokes were imaged with a TESCAN VEGA 3 scanning electron microscope (SEM). The microcrystalline parameters of various cokes in powder form were determined by X-ray diffraction (XRD) analysis using Cu Kα radiation (λ = 0.15406 nm). The crystal coherence length (La(002)) and the stacking height (Lc(002)) were calculated through a line-broadening analysis using the Scherrer equation.21 8330


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aromatic C-H stretching around 2990−3150 cm−1, aliphatic CH stretching bands around 2800−2990 cm−1, methylene C-H (probably naphthenic) in-plane bending around 1440 cm−1, and aromatic C-H deformation (out of plane bending) in the range of 700−900 cm−1,23,24 appeared in the FTIR spectra of the feedstocks, and the intensity of these absorption peaks is varying with the carbonaceous precursors. Anthracene, naphthalene synthetic pitch, petroleum-derived paving pitch, C9 aromatic hydrocarbons, and C5−C9 aromatic hydrocarbons (except coal tar-based impregnating pitch) are abundant in small moleculars (or aliphatic groups) and possess low aromatic indices (listed in Table 3). The aromatic index of coal tar-based impregnating pitch is obviously higher than those of other feedstocks, which is associated with the existence of much more polycyclic aromatic hydrocarbons (i.e., high molecular components) in the pitch. 3.2. Influence of Carbonaceous Precursors and Thermal Polymerization Temperatures on the Structure of Mesophase Pitches. The physical properties of mesophase pitch products derived from the various carbonaceous precursors thermal polymerized at different temperatures are listed in Table 2. It can be found that the main fractions of various precursor-derived mesophase pitch products vary significantly at different soaking temperatures, which leads to the marked change of softening points. The higher the heatsoaking temperatures are, the larger the heavy fractions and softening points of mesophase pitches possess. C9 aromatic hydrocarbons and petroleum-derived paving pitch possess relatively high chemical reactivity due to the existence of low molecular substances and aliphatic groups, leading to the faster increase of heavy fractions (i.e., QI) and softening points (i.e., as high as above 300 °C) than those of naphthalene synthetic pitch and coal tar-based impregnating pitch after heat-soaking under a temperature range of 410−430 °C. Naphthalene synthetic pitch unexpectedly exhibits a good characteristic of ultralow QI component after heat treatment at a temperature less than 420 °C, which is favorable for decreasing the viscosity and increasing the fluidity of molten pitch. It is well-known that the higher the softening point of a pitch is (i.e., 300 °C), the harder the pitch is melt-spun into a fiber. Therefore, it is difficult to synthesize the spinnable mesophase pitch with a suitable softening point of around 260 °C by using C9 aromatic hydrocarbons and petroleum-derived paving pitch as raw materials. According to the values of QI fraction and softening point, the thermo-chemical reactivity of anthracene and C5−C9 aromatic hydrocarbons is relatively moderate among the several feedstocks, and the reaction activity of anthracene is larger than that of C5−C9 aromatic hydrocarbons at a similar heat-soaking temperature, which has been demonstrated by the following PLM analysis. Naphthalene synthetic pitch and coal tar-based impregnating pitch show low volatilization of small molecular components and relatively high thermal stability at temperatures less than 430 °C, which is verified in the high thermal polymerization yield listed in Table 3. The low thermal polymerization yields of anthracene, petroleumderived paving pitch, C9 aromatic hydrocarbons, and C5−C9 aromatic hydrocarbons further prove the existence of large quantities of low molecular substances in the feedstocks, which have heavily volatilized during the heat-soaking process. The anisotropic liquid crystalline content of mesophase pitch products ranges from 0 to 100 vol %, which closely depends on the carbonaceous precursors and is clearly influenced by the heat-soaking temperatures. High-temper-

3. RESULTS AND DISCUSSION 3.1. Characterization of Raw Materials. The physical properties of six feedstocks are listed in Table 1. It can be found that the ash content of each feedstock is as low as hundreds of ppm, which is the basic requirement for preparing of spinnable mesophase pitch. Anthracene is a three-ring aromatic compound, and its molecular weight is small. The petroleum-derived paving pitch, C9 aromatic hydrocarbons, and C5−C9 aromatic hydrocarbons consist of many low molecular substances (the major components of C9 aromatic hydrocarbons are styrene, trimethyl- and methylethylbenzenes, indene, and polycyclene, and C5−C9 aromatic hydrocarbons are a kind of C5 modified C9 aromatic resin, which is mainly composed of pentadiene, styrene, methylstyrene- and ethylbenzene, etc.22), because the soluble light fractions (i.e., HS and HI-TS) consisting mainly of low molecular components are very high, which results in relatively low coking values under an unpressurized reaction condition. In comparison, naphthalene synthetic pitch and coal tar-based impregnating pitch possess some heavy fractions (i.e., TI-QS) and high coking values. The softening point of each feedstock (some are measured to be slightly low due to the existence of impurity) seems to be not closely related to the main fraction and coking value owing to the variety of feedstock source ranging from simple aromatic compound (i.e., anthracene) to complex aromatic hydrocarbon mixture (i.e., pitch). The various physical properties of feedstocks will undoubtedly result in the diverse characteristics of thermo-chemical reactivity, which partially dominates the formation and development of liquid crystalline mesophase. The FTIR spectra of the various carbonaceous feedstocks are shown in Figure 1. The characteristic absorption peaks, i.e.,

Figure 1. FTIR spectra of the various carbonaceous precursors (Line a: anthracene; Line b: naphthalene synthetic pitch; Line c: coal tarbased impregnating pitch; Line d: petroleum-derived paving pitch; Line e: C9 aromatic hydrocarbons; Line f: C5−C9 aromatic hydrocarbons). 8331


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Table 2. Physical Properties of Mesophase Pitches Derived from Various Carbonaceous Precursors at Different Soaking Temperatures main fraction/wt %a feedstock

soaking temperature/°C

TS

TI-QS

QI

softening point/°C

anisotropic content/vol %b

410 420 430 410 420 430 440 450 410 420 430 440 400 410 420 430 400 410 420 430 410 420 430 440 450

33.45 23.56 9.55 66.48 45.36 33.65 21.71 16.94 58.11 51.83 41.80 29.63 68.91 11.61 3.04 2.65 37.84 32.12 17.68 9.09 63.03 60.50 52.67 37.62 9.94

42.85 32.02 24.06 33.16 53.90 47.50 35.36 21.83 30.23 24.56 22.13 20.45 23.27 18.08 23.05 1.23 29.47 27.09 4.41 1.10 29.77 23.80 26.65 38.07 10.01

23.70 44.42 66.39 0.36 0.74 18.85 42.93 61.23 11.66 23.61 36.07 49.92 7.82 70.31 73.91 96.12 32.69 40.79 77.91 89.81 7.20 15.70 20.68 24.31 80.05

95 240 280 105 139 148 197 246 134 143 180 300 146 >300 >300 >300 270 >300 >300 >300 146 173 207 275 >300

0 17 100 0 3 6 12 65 0 2 7 91 26 55 76 81 15 38 56 68 0 3 18 48 95

anthracene

naphthalene synthetic pitch

coal tar-based impregnating pitch

petroleum-derived paving pitch

C9 aromatic hydrocarbons

C5−C9 aromatic hydrocarbons

a

Main fraction (i.e., toluene soluble, toluene insoluble and quinoline soluble, quinoline insoluble, marked with TS, TI-QS, and QI in sequence) was measured by solvent extraction treatments using toluene and quinolone. bAnisotropic content was image-counted by PLM.

Table 3. Thermal Polymerization Yields and Aromatic Indices of Mesophase Pitches Derived from Various Feedstocks Heat Treatment at 420 °C for 4 h mesophase pitches derived from various feedstocks parameter

anthracene






yield/wt %a Iar value/%b

1.14 0.65 (0)

69.19 0.50 (0.26)

81.70 0.75 (0.55)

9.28 0.35 (0.10)

11.70 0.26 (0.12)

8.47 0.36 (0.10)

a Yield was calculated by dividing the mass of mesophase pitch product by the total amount of the feedstock. bIar value was aromatic index calculated by the formula Iar = A3150−2990/(A3150−2990 + A2990−2800);24 the data in the brackets are those of corresponding feedstock.

ature soaking treatment benefits the increase of liquid crystalline content. The liquid crystalline content of mesophase pitches also reflects the thermo-chemical reactivity of various carbonaceous precursors. To some extent, the higher reaction activity the pitch precursor possesses, the larger the liquid crystalline content of mesophase pitch is. The FTIR spectra of the various mesophase pitches derived from different carbonaceous precursors heat treatment at 420 °C for 4 h are shown in Figure 2. It can be seen that the characteristic absorption peaks of the mesophase pitch products, i.e., aromatic C-H stretching around 2990−3150 cm−1 and aromatic C-H deformation in the range of 700−900 cm−1, increase obviously after the heat-soaking treatment, which is in agreement with the increase of aromatic indices listed in Table 3. The aromatic indices of mesophase pitch products derived from C9 aromatic hydrocarbons and petroleum-derived paving pitch are very low, which may be related to the fast thermo-chemical reaction of raw materials and lead to the limited condensation and growth of planar

aromatic rings. Anthracene, naphthalene synthetic pitch, and coal tar-based impregnating pitch possess relatively high aromatic indices (listed in Table 3). The aromatic index of mesophase pitch derived from C5−C9 aromatic hydrocarbons is very low, which is associated with its large volume of low molecular components. In general, low condensation degree and high aromaticity are important indices of high-quality mesophase pitch.6 Typical PLM micrographs of the various mesophase pitch products derived from different carbonaceous precursors heat treatment at 420 °C for 4 h are shown in Figure 3. It can be seen that the formation and development ability of liquid crystalline mesophase is obviously different for the various feedstocks. The mesophase pitch products derived from C9 aromatic hydrocarbons and petroleum-derived paving pitch possess fine-grained mosaic and coarse-grained mosaic textures, respectively. The anisotropic liquid crystalline proportions are about 50−80 vol % as shown in Figure 3e,d. The formation and transformation of liquid crystalline 8332


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mesophase for the two types of feedstocks are easy and fast due to the existence of low molecular substances with relatively high chemical reactivity. However, it is undesirable to form a bulk mesophase with a regular texture owing to the lack of (or incomplete) coalescence and deformation of liquid crystalline spheres. Decreasing the heat-soaking temperatures from 420 to 400 °C, local inhomogeneous anisotropic liquid crystalline (i.e., growth and coalescence of spheres) and sporadic mesophase spheres as shown in Figure 4 are present in the

Figure 4. Typical PLM micrographs of the various mesophase pitch products derived from (a) petroleum-derived paving pitch and (b) C9 aromatic hydrocarbons heat treatment at 400 °C for 4 h.

mesophase pitch products prepared from petroleum-derived paving pitch and C9 aromatic hydrocarbons. Their physical properties (i.e., high softening points up to 300 °C, non-whole anisotropic content) listed in Table 2 are not easily controlled because they are very sensitive to the heat-soaking temperatures. Therefore, C9 aromatic hydrocarbons and petroleumderived paving pitch seem to be unsuitable as precursors for preparing high-quality mesophase pitch (unless C9 aromatic hydrocarbons were pretreated through CH2 coupling treatment and the solvent-separated fractions were used as a feedstock25). Anthracene, naphthalene synthetic pitch, coal tar-based impregnating pitch, and C5−C9 aromatic hydrocarbons exhibit an initial stage (or period) of mesophase formation behavior (i.e., generation and growth of anisotropic liquid crystalline spheres with different sizes in a continuous isotropic phase)26 as shown in Figure 3a−c,f under a soaking temperature of 420 °C for 4 h. The liquid crystalline spheres appeared in the mesophase pitch products from naphthalene synthetic pitch, coal tar-based impregnating pitch, and C5−C9 aromatic hydrocarbons are sporadic and small, i.e., optically anisotropic small spherules suspended in the optically isotropic pitch matrix. By comparison, the liquid crystalline spheres in anthracene-derived mesophase pitch are abundant and the development of large diameter spheres is very fast. It is shown that anthracene seems to exhibit a good manner of easy formation and development of liquid crystalline spheres under the same soaking condition. Increasing the heat-soaking temperature from 420 to 430 °C, bulk liquid crystalline mesophase with a flow texture is present in the anthracene-derived mesophase pitch as shown in Figure 5a, and the anisotropic content sharply increases from 17 to 100 vol %. It is obvious that the formation and development of bulk liquid crystalline mesophase has experienced a process of generation, growth, coalescence, deformation, and disintegration of mesophase spheres upon heat treatment.27,28 The liquid crystalline spheres have enough time (4 h) to complete the conversion process (i.e., nucleation, growth, coalescence, deformation, and disintegration) at such a high temperature (providing essential energy).7,10,29 Com-

Figure 2. FTIR spectra of the various mesophase pitches derived from different carbonaceous precursors (Line a: anthracene; Line b: naphthalene synthetic pitch; Line c: coal tar-based impregnating pitch; Line d: petroleum-derived paving pitch; Line e: C9 aromatic hydrocarbons; Line f: C5−C9 aromatic hydrocarbons) heat treatment at 420 °C for 4 h.

Figure 3. Typical PLM micrographs of the various mesophase pitches derived from different carbonaceous precursors (a: anthracene; b: naphthalene synthetic pitch; c: coal tar-based impregnating pitch; d: petroleum-derived paving pitch; e: C9 aromatic hydrocarbons; f: C5− C9 aromatic hydrocarbons) heat treatment at 420 °C for 4 h.

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from naphthalene synthetic pitch is mainly taken on an inhomogeneous growth of spherical liquid crystalline as shown in Figure 6a. However, coal tar-based impregnating pitch has mostly formed a nearly bulk liquid crystalline mesophase as shown in Figure 6b; the softening point of the mesophase pitch product is about 300 °C, which indicates that coal tar-based impregnating pitch (its narrow solvent-separated components or fractions are not included) is also unsuitable to be used as precursors for preparing high-quality mesophase pitch with a relatively low softening point. The inhomogeneous liquid crystalline spheres in mesophase pitch derived from C5−C9 aromatic hydrocarbons obviously grow and coalesce as shown in Figure 6c in comparison with the micrograph shown in Figure 5d. With the further increase of the soaking temperature up to 450 °C, naphthalene synthetic pitch still exhibits a growing and coalescing behavior of liquid crystalline spheres as shown in Figure 7a. The shape and the size of the liquid Figure 5. Typical PLM micrographs of the various mesophase pitches derived from different carbonaceous precursors (a: anthracene; b: naphthalene synthetic pitch; c: coal tar-based impregnating pitch; d: C5−C9 aromatic hydrocarbons) heat treatment at 430 °C for 4 h.

bined with the acceptable softening point of 280 °C, anthracene can undoubtedly be used as a suitable precursor for preparing high-quality mesophase pitch heat treatment at a temperature range of 420 and 430 °C, which agrees well with the previous studies in the literature.29,30 In contrast, the mesophase pitch products prepared from naphthalene synthetic pitch, coal tar-based impregnating pitch, and C5− C9 aromatic hydrocarbons still exhibit an initial mesophase behavior as shown in Figure 5b−d. The optical texture of C5− C9 aromatic hydrocarbons-derived mesophase pitch heatsoaked at 430 °C is similar to that of anthracene-derived mesophase pitch heat-soaked at 420 °C, which implies that the bulk mesophase will be formed if further increasing the soaking temperatures (or prolonging the soaking time). By the same token, naphthalene synthetic pitch and coal tar-based impregnating pitch also need to increase the heat-soaking temperature to perform the conversion of liquid crystalline from the spheres to bulk mesophase. Nevertheless, excessive thermal polymerization temperature above 430 °C poses a risk of forming an insoluble and infusible coke, which needs to operate carefully and control precisely the process parameters in order to obtain mesophase pitch with high liquid crystalline content and suitable softening point.31 Figure 6 shows the typical PLM micrographs of the three types of mesophase pitch products derived from naphthalene synthetic pitch, coal tar-based impregnating pitch, and C5−C9 aromatic hydrocarbons heat treatment at 440 °C for 4 h. It can be seen that the optical texture of mesophase pitches derived

Figure 7. Typical PLM micrographs of the mesophase pitches derived from (a) naphthalene synthetic pitch and (b) C5−C9 aromatic hydrocarbons heat treatment at 450 °C for 4 h.

crystalline spheres are inhomogeneous; some round and oval shaped spheres are as large as 400−500 μm in diameter, which displays that naphthalene synthetic pitch possesses a good coalescence and plastic deformation ability. Its streamline optical texture with a bulk mesophase has finally formed after heat-soaking treatment at 460 °C for 4 h (the PLM micrograph is not shown). A good flow texture of bulk liquid crystalline mesophase as shown in Figure 7b appears in the C5−C9 aromatic hydrocarbons-derived mesophase pitch. However, its softening point is higher than 300 °C, which implies that it has little advantage of synthesizing high-quality spinnable mesophase pitch by using C5−C9 aromatic hydrocarbons as a precursor under an unpressurized reaction condition. In contrast, naphthalene synthetic pitch-derived mesophase pitch soaked at this temperature possesses a relatively low softening point of around 250 °C; however, the anisotropic mesophase content is about 65 vol % as shown in Figure 7a. Its optimum condition for the formation and development of bulk

Figure 6. Typical PLM micrographs of the various mesophase pitches derived from different carbonaceous precursors (a: naphthalene synthetic pitch; b: coal tar-based impregnating pitch; c: C5−C9 aromatic hydrocarbons) heat treatment at 440 °C for 4 h. 8334


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Figure 8. Schematic illustration of the formation and development process of bulk liquid crystalline mesophase under a suitable condition: I generation of anisotropic spheres in isotropic matrix, II growth of anisotropic spheres in isotropic matrix, III coalescence of anisotropic spheres in isotropic matrix, IV deformation and disintegration to form bulk liquid crystalline mesophase.

performed in the following study that the pressurized or catalytically thermal polymerization of the above-mentioned feedstocks with the ability of forming good mesophase texture and high anisotropic content (i.e., anthracene, naphthalene synthetic pitch, and C5−C9 aromatic hydrocarbons) in a closed vessel at a relatively low temperature less than 430 °C so as to adjust the system viscosity and improve the quality and yield of mesophase. 3.3. Structure of Various Mesophase Pitch-Derived Cokes. Typical PLM micrographs of the various mesophase pitch-derived cokes prepared from different carbonaceous precursors are shown in Figure 9. It can be clearly seen that the optical texture of various mesophase pitch-derived cokes is varying with the carbonaceous precursors.34 According to the definitions of the nomenclature to describe the size and the shape of the optical texture in the polished surfaces of anisotropic cokes,6 very fine-grained mosaic and coarse-grained mosaic textures have, respectively, presented in the cokes derived from C9 aromatic hydrocarbons and petroleum-derived paving pitch as shown in Figure 9e,d. The fine-structured coke exhibits a high isotropic degree and a very low porosity. The cokes derived from coal tar-based impregnating pitch and C5− C9 aromatic hydrocarbons are similar in macroscopic scale and both possess a supra mosaic texture mingled with local flowinduced orientation domains as shown in Figure 9c,f. The flowability of carbonaceous pitch at a molten state and the volatilization of light fractions (i.e., bubbling effect) have an obvious influence on the microstructure and crystal orientation of the resultant cokes. Good needle-like and flow-induced anisotropic textures with a high anisotropic degree are clearly found in the cokes derived from anthracene and naphthalene synthetic pitches as shown in Figure 9a,b, which could definitely promote the formation of graphitizing carbon and favor the development of a graphite-like structure with preferred orientation upon graphitization treatment. The naphthalene synthetic pitch derived coke has a porous characteristic owing to the good foaming property of the precursor. The well-oriented texture in the two types of cokes is closely related to the formation and development of flowtype liquid crystalline mesophase during the process of liquid

liquid crystalline mesophase is not easy to tackle in comparison with other carbonaceous precursors. On the basis of the size and the shape of optical textures of various mesophase pitch products from different carbonaceous precursors as shown in Figures 3−7, it can be concluded that some carbonaceous precursors (e.g., anthracene, naphthalene synthetic pitch, and C5−C9 aromatic hydrocarbons, etc.) have undergone four stages of liquid crystalline sphere development and transformation, and finally formed a bulk liquid crystalline mesophase from an isotropic matrix as illustrated in Figure 8 under a suitable reaction condition (i.e., heat-soaking temperature and time).6,7 The thermal polycondensation mechanism of various aromatic hydrocarbons or pitch materials is an intricate research subject owing to the fact that their complicated components decompose asynchronously at various temperatures.31−33 More systematic characterization needs to be performed in future work so as to deepen the understanding of the liquid carbonization process of pitch materials and realize the controllable preparation of highquality mesophase pitch. It is interesting to note that the liquid crystalline transformation of C5−C9 aromatic hydrocarbons is unexpectedly similar to that of naphthalene synthetic pitch, displaying a difficult and lagging behavior of the liquid crystalline development. In addition, there is a remarkable difference in the formation and development of liquid crystalline mesophase for C5−C9 and C9 aromatic hydrocarbons, which may be associated with the differentia in their physical and chemical constituents and molecular structures. The C5 aliphatic hydrocarbon feedstock mainly possessed a low molecular weight component that probably acts as an effective solvent or plasticizer and decreases the viscosity of molten pitch and improves its fluidity (mobility), furthermore prolonging the relatively stable thermoplastic period, which facilitates the contact coalescence and plastic deformation of anisotropic spheres.6,10 It can be concluded that the degree of coalescence of growth units of mesophase is governed by the viscosity of the various systems of mesophase pitch. The fluidity of the soaking system is therefore enhanced, allowing the development of anisotropic carbon (of larger optical texture). It will be 8335


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Figure 9. Typical PLM micrographs of the various mesophase pitchderived cokes prepared from different carbonaceous precursors (a: anthracene; b: naphthalene synthetic pitch; c: coal tar-based impregnating pitch; d: petroleum-derived paving pitch; e: C9 aromatic hydrocarbons; f: C5−C9 aromatic hydrocarbons) heat treatment at 900 °C for 1 h.

phase carbonization of the precursors in a suitable temperature range of 400−450 °C. Typical SEM images of broken surfaces of the various mesophase pitch-derived cokes prepared from different carbonaceous precursors as shown in Figure 10 well demonstrate the microcrystal (i.e., carbon layer) size and crystal orientation of the various cokes varying from a fine grained texture to an ordered lamellar texture, which is in agreement with the corresponding optical texture. That is to say, the structure of cokes inherits that of the mesophase pitches, and there is great potential to tailor their structure during the liquid-crystal phase of the synthesizing process.35 The size, growth, and coalescence characteristics of the mesophase spherules influence very closely the structure (and physical properties) of the final carbon and graphite products. It is interesting to note that a few carbon microspheres clearly appear on the broken surface of the mesophase pitch-derived cokes as shown in Figure 10b,d, which is further exhibited in Figure 11a. The integrated existence of carbon microspheres and oriented carbon layers is associated with the unsynchronized and inhomogeneous conversion of liquid crystalline optically anisotropic spheres (i.e., growth, coalescence, and deformation and orientation) in an isotropic matrix as shown in Figure 11b, which is consistent with the gradual formation and development process of bulk liquid crystalline mesophase as shown in Figure 8. That is to say, it is not easy to obtain a 100 vol % anisotropic mesophase pitch with a streamline texture, which depends on the carbonaceous precursor and thermal reaction conditions. It is concluded that the carbonaceous precursors have a significant effect on the anisotropic content and optical texture

Figure 10. Typical SEM images of broken surfaces of the various mesophase pitch-derived cokes prepared from different carbonaceous precursors (a: anthracene; b: naphthalene synthetic pitch; c: coal tarbased impregnating pitch; d: petroleum-derived paving pitch; e: C9 aromatic hydrocarbons; f: C5−C9 aromatic hydrocarbons) heat treatment at 900 °C for 1 h.

of the mesophase pitches as discussed above, which undoubtedly results in the microstructure variety of the resultant cokes. This, in turn, leads to the suggestion that cokes with various optical textures of domains are formed from the complex process of liquid phase carbonization in which the formation and development abilities of the liquid crystalline mesophase for different carbonaceous precursors have a significant role. The complete conversion from isotropic pitch matrix to bulk liquid crystalline mesophase is intractable, which is mainly influenced by some of the following factors, e.g., molecular size, molecular constituent and thermochemical reactivity of the carbonaceous precursors, viscosity and fluidity of the molten system, heat treatment temperature and time, etc.32 Figure 12 shows XRD spectra of the various mesophase pitch-derived cokes prepared from different carbonaceous precursors heat treatment at 900 °C for 1 h. The XRD profiles of the various mesophase pitch-derived cokes show a similar pattern of one broad diffraction peak at around 2θ = 25.4° 8336


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Figure 11. (a) SEM image of broken surface of mesophase pitch-derived coke and (b) PLM micrograph of the mesophase pitch showing an unsynchronized and inhomogeneous conversion of liquid crystalline optically anisotropic spheres in isotropic matrix.

XRD analysis result is well consistent with the optical texture and microstructure analyses as shown in Figures 9 and 10, as anticipated above.

4. CONCLUSION This work clearly demonstrates the effect of carbonaceous precursors on the structure of mesophase pitch products and their derived cokes. The thermo-chemical reactivity and reaction rate of forming liquid crystalline mesophase are impressively diverse for various carbonaceous precursors at different soaking temperatures, which results in the differences in the anisotropic content and optical texture of resultant mesophase pitch products. Under the same soaking condition (i.e., in an unpressurized circumstance), the formation and transformation sequence of liquid crystalline mesophase for the six different feedstocks is as follows: C9 aromatic hydrocarbons, petroleum-derived paving pitch, anthracene, coal tar-based impregnating, C5−C9 aromatic hydrocarbons, and naphthalene synthetic pitch. Anthracene is desirable to form a bulk mesophase with a good streamline texture under a mild condition, which provides a possibility for preparing spinnable mesophase pitch easily. However, it is difficult to form a bulk liquid crystalline mesophase from naphthalene synthetic pitch or C5−C9 aromatic hydrocarbons under the similar condition. Bulk liquid crystalline mesophase has undergone the four-stage developing process of liquid crystalline spheres (i.e., generation, growth, coalescence, deformation, and disintegration) and gradually transformed from the isotropic matrix. C9 aromatic hydrocarbons, petroleum-derived paving pitch, coal tar-based impregnating, and C5−C9 aromatic hydrocarbons form various mosaic-structured cokes with different degrees from a fine grained texture to a supra mosaic texture. In

Figure 12. XRD spectra of the various mesophase pitch-derived cokes prepared from different carbonaceous precursors (Line a: anthracene; Line b: naphthalene synthetic pitch; Line c: coal tar-based impregnating pitch; Line d: petroleum-derived paving pitch; Line e: C9 aromatic hydrocarbons; Line f: C5−C9 aromatic hydrocarbons) heat treatment at 900 °C for 1 h.

corresponding to (0 0 2) crystal planes of hexagonal graphite. The relative intensity of (0 0 2) diffraction peaks of the various mesophase pitch-derived cokes are different for various carbonaceous precursors. The cokes derived from C9 aromatic hydrocarbons and petroleum-derived paving pitch present relatively weaker (0 0 2) diffraction peaks than those of other mesophase pitch-derived cokes. The microcrystal parameters (La and Lc) and crystal orientation of the two types of cokes are less than others as listed in Table 4. And anthracenederived coke possesses a relatively large microcrystal size. The

Table 4. Microcrystalline Parameters of the Various Mesophase Pitch-Derived Cokes Prepared from Different Feedstocks Heat Treatment at 900 °C for 1 h mesophase pitch-derived cokes from different precursors parameter

anthracene


2θ(002)/deg d(002)/nm Lc(002)/nm La(002)/nma

25.56 0.348 2.7 7.5

25.46 0.349 2.5 7.0





25.35 0.351 1.9 6.1

25.23 0.355 1.8 4.8

24.94 0.357 1.6 4.4

25.45 0.349 2.4 7.0

La(002) value was calculated by the relation La = 9.5/(d(002) ̵ 3.354) in Å.21

a

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contrast, anthracene and naphthalene synthetic pitch-derived cokes exhibit a well-oriented texture, which is closely related to the formation and development of flow-type liquid crystalline in mesophase pitch products during the process of liquid phase carbonization.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.Y.). *E-mail: [email protected] (X.L.). ORCID

Guanming Yuan: 0000-0002-4693-8358 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the National Natural Science Foundation of China (grant No. 51372177) and the Key Laboratory of Hubei Province for Coal Conversion and New Carbon Materials (grant No. WKDM201701).

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