# Effect of Different Catalysts on Properties of Coal Tar Pitch Modified by Cinnamaldehyde

Authors: Wenjuan Zhang; Wenhong Tian; Shihua Song; Xianren Zeng; Peng Gao; Lili Mao; Tiehu Li
Journal: Journal of Spectroscopy
 (2019)

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## Abstract

Cinnamaldehyde- (CMA-) modified coal tar pitches (CTPs) are prepared in the presence of acids. In this paper, the effect of boric acid andp-toluene sulfonic acid on the pyrolysis and graphitization process of CMA-modified CTP was studied. The pyrolysis process was studied by Fourier transform infrared spectroscopy, thermogravimetric analysis and derivative thermogravimetry, and polarized-light microscopy. In addition, the graphitization process was studied by X-ray diffraction and Raman spectroscopy. The results indicate the carbon yield of CMA-modified CTP with boric acid as catalyst (B7C10) is higher than that of CMA-modified CTP with p-toluene sulfonic acid as a catalyst (P7C10). In addition, under the same experimental condition (heated at 400°C and held for 1 h), the mesophase spheres of B7C10 are more regular than those of P7C10 and the largest diameter of the mesophase spheres can reach to 40 um. Further, after the graphitization process, the graphitization degree of B7C10 is higher than that of P7C10. So, it is more effective to modify CTP with boric acid as a catalyst.


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## Body

## 1. Introduction

Coal tar pitch (CTP) is a composite material composed of aromatic compounds with wide molecular weight distribution [1]. As a low cost precursor and because of the ability to produce graphitized carbon, CTP is a promising candidate for the production of carbon materials and carbon-carbon composite materials. [2]. However, it has low carbon yield, low density, and large coking pore volume, which require a series of impregnation/carbonization steps for subsequent densification under high pressure. Therefore, it is time-consuming and expensive to treat carbon materials or carbon-carbon composites with CTP as a matrix precursor [3]. Therefore, the key to simplify and reduce the cost of the preparation process is to improve the carbon yield of CTP by specific treatment methods. [4].It is well known that the carbonization yield of CTP can be improved by physical separation and chemical modification. Physical separation can be used to separate components with different average molecular weights in CTP [5]. On the other hand, chemical modifications involve chemical reactions between CTP and polystyrene, rosin, lignin/silica hybrid, and so on [6–12]. Compared with physical separation, chemical modification has advantages in saving resources, reducing waste treatment, and simplifying the preparation process. Therefore, it has been the typical way to improve the carbonization yield of CTP.Cinnamaldehyde (CMA) is a kind of alpha, beta-unsaturated aldehyde, in which the benzene ring is conjugated with C=O and C=C groups. Therefore, it is a potential natural crosslinker [13]. Hydrogenation reactions can occur simultaneously in C=O and C=C groups [14]. In our previous study, CMA was used to modify CTP with boric acid and p-toluene sulfonic acid as catalysts, respectively [15, 16]. Higher carbonization yield and better properties of modified CTP can be obtained because of the cross-linking reaction between CMA and CTP. However, the catalysts also play important roles in the modification of CTP with CMA. In this paper, we investigate the effect of the two catalysts on the pyrolysis and graphitization process of CMA-modified CTP.


## 2. Experiment

### 2.1. Materials

CTP was purchased from the Steel Co. Ltd. (Wuhan, China). CMA, toluene, quinoline, boric acid, and PTS were of analytical grade. Some properties of the CTP are included in Table1.Table 1
Physical properties of the parent CTP and two CMA-modified CTPs.
SP
CV
TI
QI
Elemental analysis (wt.%)
C/H
(°C)
(wt.%)
(wt.%)
(wt.%)
N
C
H
B0C0
120
57.75
55.95
7.90
2.50
93.07
4.09
1.90
B7C10
114
67.58
46.42
14.62
1.78
85.46
4.29
1.66
P7C10
112
65.02
51.54
11.24
0.64
89.82
3.99
1.88
SP, softening point; CV, coking value; TI, toluene insolubles; QI, quinoline insolubles.


### 2.2. Modification of CTP

In our previous research, the optimal experimental condition for CMA-modified CTP is 100 g CTP and 7 g PTS mixed with 10 ml of CMA [16]. For comparison, CMA-modified CTPs with boric acid and p-toluene sulfonic acid as catalysts were prepared under the same experimental conditions. The detailed preparation method was described in references [15, 16]. The pitches studied in this paper are the parent CTP (B0C0), the CTP modified with 10 ml of CMA with 7 g of boric acid as a catalyst (B7C10), and the CTP modified with 10 ml of CMA with 7 g of PTS as a catalyst (P7C10), respectively.


### 2.3. Preparation of Mesocarbon Microbeads

The mesophase spheres of the parent CTP and modified CTPs were prepared in a high-pressure reaction kettle. About 5 g of the sample was heated to 400°C at a heating rate of 1°C·min−1 and kept for 1 h at this temperature. In the whole process, nitrogen gas was introduced into the kettle.


### 2.4. Graphitization of CTPs

The graphitization experiment was carried out in a graphitizing furnace. First, the samples were carbonized at 900°C. Then, the carbonized samples were placed in a graphite crucible and heated to 2400°C at a heating rate of 10°C·min−1 for 2 h. The resultant graphitized products of B0C0, B7C10, and P7C10 were denoted as B0C0–2400, B7C10–2400, and P7C10–2400, respectively.


### 2.5. Characteristics of CTP and CMA-Modified CTPs

The compositions of CTP and CMA-modified CTPs were analyzed by standard methods: softening point (SP), ring and ball method, ASTM D36-66; coking value (CV), ISO 6998; toluene insolubles (TI), ISO 6376-96; quinoline insolubles (QI), ISO 6791-81.


### 2.6. Measurements

Elemental analysis of C, H, and N was performed on a Vario EL-III analyzer. Fourier transform infrared (FT-IR) spectroscopy was acquired on a Bruker Tenser-27 FT-IR spectrometer with thin films of KBr in the range of 4000–400 cm−1. Thermogravimetric analysis (TG-DTG) was performed on a Mettler-Toledo 851e thermal analyzer under N2 atmosphere with a heating rate of 10°C·min−1.The optical textures of the semicokes were observed using an OLYMPUS-B061 polarized-light microscope. X-ray diffraction (XRD) was measured on a PANalytical X’Pert PRO X-ray diffractometer with CuKα (λ = 1.5406 Å) radiation at 40 kV and 35 mA. Raman spectra were performed on the Raman microscope (inVia, Renishaw, London, England) at 900–2000 cm−1. The spectral excitation was provided by an Ar ion laser, using the 514.5 nm line and with proper power density on the sample surface.


## 2.1. Materials

CTP was purchased from the Steel Co. Ltd. (Wuhan, China). CMA, toluene, quinoline, boric acid, and PTS were of analytical grade. Some properties of the CTP are included in Table1.Table 1
Physical properties of the parent CTP and two CMA-modified CTPs.
SP
CV
TI
QI
Elemental analysis (wt.%)
C/H
(°C)
(wt.%)
(wt.%)
(wt.%)
N
C
H
B0C0
120
57.75
55.95
7.90
2.50
93.07
4.09
1.90
B7C10
114
67.58
46.42
14.62
1.78
85.46
4.29
1.66
P7C10
112
65.02
51.54
11.24
0.64
89.82
3.99
1.88
SP, softening point; CV, coking value; TI, toluene insolubles; QI, quinoline insolubles.


## 2.2. Modification of CTP

In our previous research, the optimal experimental condition for CMA-modified CTP is 100 g CTP and 7 g PTS mixed with 10 ml of CMA [16]. For comparison, CMA-modified CTPs with boric acid and p-toluene sulfonic acid as catalysts were prepared under the same experimental conditions. The detailed preparation method was described in references [15, 16]. The pitches studied in this paper are the parent CTP (B0C0), the CTP modified with 10 ml of CMA with 7 g of boric acid as a catalyst (B7C10), and the CTP modified with 10 ml of CMA with 7 g of PTS as a catalyst (P7C10), respectively.


## 2.3. Preparation of Mesocarbon Microbeads

The mesophase spheres of the parent CTP and modified CTPs were prepared in a high-pressure reaction kettle. About 5 g of the sample was heated to 400°C at a heating rate of 1°C·min−1 and kept for 1 h at this temperature. In the whole process, nitrogen gas was introduced into the kettle.


## 2.4. Graphitization of CTPs

The graphitization experiment was carried out in a graphitizing furnace. First, the samples were carbonized at 900°C. Then, the carbonized samples were placed in a graphite crucible and heated to 2400°C at a heating rate of 10°C·min−1 for 2 h. The resultant graphitized products of B0C0, B7C10, and P7C10 were denoted as B0C0–2400, B7C10–2400, and P7C10–2400, respectively.


## 2.5. Characteristics of CTP and CMA-Modified CTPs

The compositions of CTP and CMA-modified CTPs were analyzed by standard methods: softening point (SP), ring and ball method, ASTM D36-66; coking value (CV), ISO 6998; toluene insolubles (TI), ISO 6376-96; quinoline insolubles (QI), ISO 6791-81.


## 2.6. Measurements

Elemental analysis of C, H, and N was performed on a Vario EL-III analyzer. Fourier transform infrared (FT-IR) spectroscopy was acquired on a Bruker Tenser-27 FT-IR spectrometer with thin films of KBr in the range of 4000–400 cm−1. Thermogravimetric analysis (TG-DTG) was performed on a Mettler-Toledo 851e thermal analyzer under N2 atmosphere with a heating rate of 10°C·min−1.The optical textures of the semicokes were observed using an OLYMPUS-B061 polarized-light microscope. X-ray diffraction (XRD) was measured on a PANalytical X’Pert PRO X-ray diffractometer with CuKα (λ = 1.5406 Å) radiation at 40 kV and 35 mA. Raman spectra were performed on the Raman microscope (inVia, Renishaw, London, England) at 900–2000 cm−1. The spectral excitation was provided by an Ar ion laser, using the 514.5 nm line and with proper power density on the sample surface.


## 3. Results and Discussion

### 3.1. Characteristics of the Parent CTP and Two CMA-Modified CTPs

The main characteristics of the parent CTP (B0C0) and two CMA-modified CTPs (B7C10 and P7C10) are listed in Table 1. It can be observed that B7C10 and P7C10 have lower softening points, TI content, and C/H ratio than those of B0C0. While the CV and QI content of B7C10 and P7C10 are higher than that of B0C0. The highest CV can be obtained for B7C10. So, after the modification of CTP with CMA in the presence of acid, the CV of CTP is indeed improved.


### 3.2. FT-IR Analysis

FT-IR spectra of B0C0, B7C10, and P7C10 are shown in Figure 1. The attribution of the peaks of B0C0 is described in references [15–17]. For the FT-IR spectra of B7C10 and P7C10, the disappearance of the peak at 3420 cm−1 indicates the reaction between CMA and O–H. In addition, the peaks at 1670 cm−1, 1120 cm−1, and 700 cm−1 are the characteristic peaks of CMA [18]. In the FT-IR spectrum of B7C10, the peak attributed to the O–H stretching vibration of boric acid is at 3200 cm−1, while the peak at 1192 cm−1 is due to the asymmetric stretching vibration of the tetrahedral BO4 [19]. In the spectrum of P7C10, the characteristic peaks of PTS are at 1226 cm−1, 1180 cm−1, 1035 cm−1, and 567 cm−1 [20]. The appearance of the peaks indicates the presence of the matter.Figure 1
FT-IR spectra of the parent CTP and two CMA-modified CTPs.


### 3.3. TG-DTG Analysis

The TG curves of B0C0, B7C10 and P7C10 are shown in Figure 2. The results show that both parental CTP and modified CTP decompose at a mass loss stage in the temperature range of 25–800°C. Weight loss is mainly due to the removal of light compounds and gases produced by thermal polymerization and aromatic ring side chain cracking [1]. The carbonization yield of B0C0, B7C10, and P7C10 at 800°C are 41.04%, 46.68%, and 46.12%, respectively, indicating that the carbonization yield of CTP increases after CTP is modified by CMA.Figure 2
TG curves of the parent CTP and two CMA-modified CTPs.The physical and chemical changes during the CTP pyrolysis can be well understood by the combination analysis of TG and DTG results. The DTG curves of the parent CTP and two CMA-modified CTPs are shown in Figure3. The DTG curve of B0C0 is characterized by a single peak centered at 362°C, indicating that the mass loss rate at this temperature reaches a maximum. However, in the DTG curves of B7C10 and P7C10, two small peaks appeared at a temperature of about 370°C, which may be caused by complex changes in this temperature. In addition, in the DTG curve of B7C10, the peak at about 150°C is caused by dehydration of boric acid [21]. Otherwise, at 500°C, the peak corresponding to the thermal polymerization at this temperature is more pronounced in the DTG curves of B7C10 and P7C10 than in B0C0.Figure 3
DTG curves of the parent CTP and two CMA-modified CTPs.


### 3.4. Optical Texture of Resultant Semicokes

The optical texture observed by a polarizing microscope is closely related to the conductivity, thermal expansion, mechanical strength, and graphitization properties of carbon materials and is one of the most relevant characteristics of carbon materials [22]. When CTP is heated above 350°C, mesophase spheres will appear in its optical structure, which is a good precursor of high-performance carbon materials [23].The optical structure of the product prepared from B0C0, B7C10, and P7C10 after heating at 400°C for 1 hour is shown in Figure 4. The optical micrograph of the parent CTP (Figure 4(a)) shows that some small mesophase spheres are produced. These spheres have a maximum diameter of about 5 um and are very dispersed. Figure 4(b) shows the mesophase spheres obtained from B7C10 after heating at 400°C for 1 h. Compared to B0C0 (Figure 4(a)), the number of mesophase spheres increases and the shape is more regular. These spheres have a maximum diameter of 40 um. The optical structure of P7C10 treated under the same conditions is shown in Figure 4(c). Compared to B0C0 (Figure 4(a)), the number of mesophase spheres increases, but the shapes of these balls are less regular and the size is about 15 um. From this, it can be seen that the number of mesophase spheres increases after the modification of CTP by CMA. But the B7C10 can get larger sizes and more regular spheres.Figure 4
Optical micrographs of the products prepared from B0C0, B7C10, and P7C10 heated at 400°C for 1 h. (a) B0C0; (b) B7C10; (c) P7C10.


### 3.5. XRD Analysis

To study the effect of boric acid and PTS on the graphitization process of CMA-modified coal tar pitch, B0C0–2400, B7C10–2400, and P7C10–2400, which are the graphitized products of B0C0, B7C10, and P7C10 heated at 2400°C and held for 2 h, respectively, were prepared and characterized by XRD and Raman spectra.Figures5(a)–5(c) show the XRD patterns of the three CTPs and their corresponding graphitized products. Figure 5(d) is the magnified XRD patterns of B0C0–2400, B7C10–2400, and P7C10–2400. By comparing the XRD patterns of CTP with its corresponding graphitized products (Figures 5(a)–5(c)), it can be observed that the diffraction peak centered at 25°, which is generally indexed to (002) diffraction of graphite [24], becomes sharp after graphitization while the other diffraction peaks of graphite loomed. Otherwise, in the XRD patterns of B7C10 (Figure 5(b)), the peaks centered at 14.67° and 28.1° are the diffraction peaks of boric acid [25]. To see clearly the small peaks of the graphitized products of B0C0–2400, B7C10–2400, and P7C10–2400, the XRD patterns are magnified (Figure 5(d)). It is worth noting that (201) and (114) crystal planes of graphite can be obverted in the XRD patterns of B7C10–2400 but cannot be found in the XRD patterns of B0C0–2400 and P7C10–2400, which indicates B7C10–2400 possesses a higher degree of graphitization than B0C0–2400 and P7C10–2400.Figure 5
XRD patterns of CTPs and their graphitized products.
(a)
(b)
(c)
(d)According to the XRD patterns, the crystal structure parameters (d002, the interlayer spacing; Lc, the crystallite sizes along the c-axis; G, the graphitization degree) of B0C0–2400, B7C10–2400, and P7C10–2400 are calculated by the Bragg formula and the Debye-Scherrer equation, respectively [26], and the results are listed in Table 2. It can be observed that d002 and Lc values of B7C10–2400 are smaller than those of B0C0–2400, and the graphitization degree of B7C10–2400 is larger than that of B0C0–2400. This indicates the graphitization degree of CMA-modified CTP can be increased obviously with boric acid as a catalyst. For P7C10–2400, the value of d002 is larger than that of B0C0–2400, but the values of Lc and G are smaller than those of B0C0–2400. This shows that the graphitization degree of CMA-modified CTP with PTS as a catalyst cannot be increased although the carbon yield is increased.Table 2
XRD parameters of B0C0–2400, B7C10–2400, and P7C10–2400.
2θ002 (°)
d
002 (nm)
β
002 (°)
L
c
G (%)
B0C0–2400
26.376
0.3376
0.5651
14.2786
74.4
B7C10–2400
26.516
0.3359
0.6169
13.0834
94.2
P7C10–2400
26.277
0.3389
0.8042
10.0313
59.3
d
002 = λ/2sinθ002; Lc = 0.89λ/β002⋅cosθ002; β is the radian of full width at half maximum intensity (FWHM in radian); λ = 0.15406 nm; G=0.3440−d002/0.3440−0.3354.


### 3.6. Raman Analysis

The Raman spectrum of a single graphite crystal usually shows a very strong peak corresponding to the E2G mode at 1575 cm−1 (G band). Polycrystalline graphite and disordered carbon exhibit additional peaks at 1355 cm−1 (D band). The IG/ID intensity ratio is considered to be an indicator of the degree of sample disorder. The larger the IG/ID value, the lower the degree of disorder [27].Figure6 shows the Raman spectra of B0C0–2400, B7C10–2400, and P7C10–2400. The two peaks of P7C10–2400 is little different from that of B0C0–2400. But for B7C10–2400, the two peaks shift to higher frequency and the intensity increases strongly.Figure 6
Raman spectra of B0C0–2400, B7C10–2400, and P7C10–2400.Table3 lists the IG/ID value of Raman spectra of B0C0–2400, B7C10–2400, and P7C10–2400. It is obvious that B7C10–2400 has higher IG/ID value than B0C0–2400 and P7C10–2400, which indicates B7C10–2400 has a higher degree of graphitization. So, the result of the Raman spectroscopy is consistent with the XRD.Table 3
Raman spectrum parameters of B0C0–2400, B7C10–2400, and P7C10–2400.
B0C0–2400
B7C10–2400
P7C10–2400
I
G/ID
3.19
3.76
3.13From the XRD and Raman spectroscopy analysis, it can be concluded that B7C10 has a higher graphitization degree than P7C10. The reason may be due to the good compatibility between boron atoms and carbon atoms. The covalent radius of boron atoms and carbon atoms is 0.088 nm and 0.077 nm, respectively. In addition, the diffusion coefficient of boron atoms in the direction of the graphite crystal is as high as 6320 cm2/S−1 [28]. Therefore, it is possible that boron atoms may occupy the disordered carbon structure by the diffusion of solid solution, and the defects of the disordered structure are eliminated [29]. So, the modified CTP with boric acid as a catalyst has a higher graphitization degree than that with p-toluene sulfonic acid as a catalyst.


## 3.1. Characteristics of the Parent CTP and Two CMA-Modified CTPs

The main characteristics of the parent CTP (B0C0) and two CMA-modified CTPs (B7C10 and P7C10) are listed in Table 1. It can be observed that B7C10 and P7C10 have lower softening points, TI content, and C/H ratio than those of B0C0. While the CV and QI content of B7C10 and P7C10 are higher than that of B0C0. The highest CV can be obtained for B7C10. So, after the modification of CTP with CMA in the presence of acid, the CV of CTP is indeed improved.


## 3.2. FT-IR Analysis

FT-IR spectra of B0C0, B7C10, and P7C10 are shown in Figure 1. The attribution of the peaks of B0C0 is described in references [15–17]. For the FT-IR spectra of B7C10 and P7C10, the disappearance of the peak at 3420 cm−1 indicates the reaction between CMA and O–H. In addition, the peaks at 1670 cm−1, 1120 cm−1, and 700 cm−1 are the characteristic peaks of CMA [18]. In the FT-IR spectrum of B7C10, the peak attributed to the O–H stretching vibration of boric acid is at 3200 cm−1, while the peak at 1192 cm−1 is due to the asymmetric stretching vibration of the tetrahedral BO4 [19]. In the spectrum of P7C10, the characteristic peaks of PTS are at 1226 cm−1, 1180 cm−1, 1035 cm−1, and 567 cm−1 [20]. The appearance of the peaks indicates the presence of the matter.Figure 1
FT-IR spectra of the parent CTP and two CMA-modified CTPs.


## 3.3. TG-DTG Analysis

The TG curves of B0C0, B7C10 and P7C10 are shown in Figure 2. The results show that both parental CTP and modified CTP decompose at a mass loss stage in the temperature range of 25–800°C. Weight loss is mainly due to the removal of light compounds and gases produced by thermal polymerization and aromatic ring side chain cracking [1]. The carbonization yield of B0C0, B7C10, and P7C10 at 800°C are 41.04%, 46.68%, and 46.12%, respectively, indicating that the carbonization yield of CTP increases after CTP is modified by CMA.Figure 2
TG curves of the parent CTP and two CMA-modified CTPs.The physical and chemical changes during the CTP pyrolysis can be well understood by the combination analysis of TG and DTG results. The DTG curves of the parent CTP and two CMA-modified CTPs are shown in Figure3. The DTG curve of B0C0 is characterized by a single peak centered at 362°C, indicating that the mass loss rate at this temperature reaches a maximum. However, in the DTG curves of B7C10 and P7C10, two small peaks appeared at a temperature of about 370°C, which may be caused by complex changes in this temperature. In addition, in the DTG curve of B7C10, the peak at about 150°C is caused by dehydration of boric acid [21]. Otherwise, at 500°C, the peak corresponding to the thermal polymerization at this temperature is more pronounced in the DTG curves of B7C10 and P7C10 than in B0C0.Figure 3
DTG curves of the parent CTP and two CMA-modified CTPs.


## 3.4. Optical Texture of Resultant Semicokes

The optical texture observed by a polarizing microscope is closely related to the conductivity, thermal expansion, mechanical strength, and graphitization properties of carbon materials and is one of the most relevant characteristics of carbon materials [22]. When CTP is heated above 350°C, mesophase spheres will appear in its optical structure, which is a good precursor of high-performance carbon materials [23].The optical structure of the product prepared from B0C0, B7C10, and P7C10 after heating at 400°C for 1 hour is shown in Figure 4. The optical micrograph of the parent CTP (Figure 4(a)) shows that some small mesophase spheres are produced. These spheres have a maximum diameter of about 5 um and are very dispersed. Figure 4(b) shows the mesophase spheres obtained from B7C10 after heating at 400°C for 1 h. Compared to B0C0 (Figure 4(a)), the number of mesophase spheres increases and the shape is more regular. These spheres have a maximum diameter of 40 um. The optical structure of P7C10 treated under the same conditions is shown in Figure 4(c). Compared to B0C0 (Figure 4(a)), the number of mesophase spheres increases, but the shapes of these balls are less regular and the size is about 15 um. From this, it can be seen that the number of mesophase spheres increases after the modification of CTP by CMA. But the B7C10 can get larger sizes and more regular spheres.Figure 4
Optical micrographs of the products prepared from B0C0, B7C10, and P7C10 heated at 400°C for 1 h. (a) B0C0; (b) B7C10; (c) P7C10.


## 3.5. XRD Analysis

To study the effect of boric acid and PTS on the graphitization process of CMA-modified coal tar pitch, B0C0–2400, B7C10–2400, and P7C10–2400, which are the graphitized products of B0C0, B7C10, and P7C10 heated at 2400°C and held for 2 h, respectively, were prepared and characterized by XRD and Raman spectra.Figures5(a)–5(c) show the XRD patterns of the three CTPs and their corresponding graphitized products. Figure 5(d) is the magnified XRD patterns of B0C0–2400, B7C10–2400, and P7C10–2400. By comparing the XRD patterns of CTP with its corresponding graphitized products (Figures 5(a)–5(c)), it can be observed that the diffraction peak centered at 25°, which is generally indexed to (002) diffraction of graphite [24], becomes sharp after graphitization while the other diffraction peaks of graphite loomed. Otherwise, in the XRD patterns of B7C10 (Figure 5(b)), the peaks centered at 14.67° and 28.1° are the diffraction peaks of boric acid [25]. To see clearly the small peaks of the graphitized products of B0C0–2400, B7C10–2400, and P7C10–2400, the XRD patterns are magnified (Figure 5(d)). It is worth noting that (201) and (114) crystal planes of graphite can be obverted in the XRD patterns of B7C10–2400 but cannot be found in the XRD patterns of B0C0–2400 and P7C10–2400, which indicates B7C10–2400 possesses a higher degree of graphitization than B0C0–2400 and P7C10–2400.Figure 5
XRD patterns of CTPs and their graphitized products.
(a)
(b)
(c)
(d)According to the XRD patterns, the crystal structure parameters (d002, the interlayer spacing; Lc, the crystallite sizes along the c-axis; G, the graphitization degree) of B0C0–2400, B7C10–2400, and P7C10–2400 are calculated by the Bragg formula and the Debye-Scherrer equation, respectively [26], and the results are listed in Table 2. It can be observed that d002 and Lc values of B7C10–2400 are smaller than those of B0C0–2400, and the graphitization degree of B7C10–2400 is larger than that of B0C0–2400. This indicates the graphitization degree of CMA-modified CTP can be increased obviously with boric acid as a catalyst. For P7C10–2400, the value of d002 is larger than that of B0C0–2400, but the values of Lc and G are smaller than those of B0C0–2400. This shows that the graphitization degree of CMA-modified CTP with PTS as a catalyst cannot be increased although the carbon yield is increased.Table 2
XRD parameters of B0C0–2400, B7C10–2400, and P7C10–2400.
2θ002 (°)
d
002 (nm)
β
002 (°)
L
c
G (%)
B0C0–2400
26.376
0.3376
0.5651
14.2786
74.4
B7C10–2400
26.516
0.3359
0.6169
13.0834
94.2
P7C10–2400
26.277
0.3389
0.8042
10.0313
59.3
d
002 = λ/2sinθ002; Lc = 0.89λ/β002⋅cosθ002; β is the radian of full width at half maximum intensity (FWHM in radian); λ = 0.15406 nm; G=0.3440−d002/0.3440−0.3354.


## 3.6. Raman Analysis

The Raman spectrum of a single graphite crystal usually shows a very strong peak corresponding to the E2G mode at 1575 cm−1 (G band). Polycrystalline graphite and disordered carbon exhibit additional peaks at 1355 cm−1 (D band). The IG/ID intensity ratio is considered to be an indicator of the degree of sample disorder. The larger the IG/ID value, the lower the degree of disorder [27].Figure6 shows the Raman spectra of B0C0–2400, B7C10–2400, and P7C10–2400. The two peaks of P7C10–2400 is little different from that of B0C0–2400. But for B7C10–2400, the two peaks shift to higher frequency and the intensity increases strongly.Figure 6
Raman spectra of B0C0–2400, B7C10–2400, and P7C10–2400.Table3 lists the IG/ID value of Raman spectra of B0C0–2400, B7C10–2400, and P7C10–2400. It is obvious that B7C10–2400 has higher IG/ID value than B0C0–2400 and P7C10–2400, which indicates B7C10–2400 has a higher degree of graphitization. So, the result of the Raman spectroscopy is consistent with the XRD.Table 3
Raman spectrum parameters of B0C0–2400, B7C10–2400, and P7C10–2400.
B0C0–2400
B7C10–2400
P7C10–2400
I
G/ID
3.19
3.76
3.13From the XRD and Raman spectroscopy analysis, it can be concluded that B7C10 has a higher graphitization degree than P7C10. The reason may be due to the good compatibility between boron atoms and carbon atoms. The covalent radius of boron atoms and carbon atoms is 0.088 nm and 0.077 nm, respectively. In addition, the diffusion coefficient of boron atoms in the direction of the graphite crystal is as high as 6320 cm2/S−1 [28]. Therefore, it is possible that boron atoms may occupy the disordered carbon structure by the diffusion of solid solution, and the defects of the disordered structure are eliminated [29]. So, the modified CTP with boric acid as a catalyst has a higher graphitization degree than that with p-toluene sulfonic acid as a catalyst.


## 4. Conclusions

The effect of boric acid andp-toluene sulfonic acid on the pyrolysis and graphitization process of CMA-modified CTP was compared. The results show that larger size and more regular mesophase spheres can be obtained from B7C10 compared with P7C10 under the same experimental condition. Furthermore, the product of B7C10 graphitized at 2400°C possesses a higher graphitization degree than that of P7C10. Therefore, boric acid is better than p-toluene sulfonic acid as a catalyst in the modification of CTP.



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