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

Cinnamaldehyde(CMA-) modified coal tar pitches (CTPs) are prepared in the presence of acids. In this paper, the effect of boric acid and p-toluene sulfonic acid on the pyrolysis and graphitization process of CMA-modified CTP was studied. -e 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. -e results indicate the carbon yield of CMA-modified CTP with boric acid as catalyst (B7C10) is higher than that of CMAmodified 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.


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 carboncarbon 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.erefore, it is time-consuming and expensive to treat carbon materials or carbon-carbon composites with CTP as a matrix precursor [3].erefore, 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][7][8][9][10][11][12].Compared with physical separation, chemical modification has advantages in saving resources, reducing waste treatment, and simplifying the preparation process.erefore, it has been the typical way to improve the carbonization yield of CTP.
Cinnamaldehyde (CMA) is a kind of alpha, betaunsaturated aldehyde, in which the benzene ring is conjugated with C�O and C�C groups.erefore, 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.

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 Table 1.

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 ptoluene sulfonic acid as catalysts were prepared under the same experimental conditions.e detailed preparation method was described in references [15,16].e pitches studied in this paper are the parent CTP (B 0 C 0 ), the CTP modified with 10 ml of CMA with 7 g of boric acid as a catalyst (B 7 C 10 ), and the CTP modified with 10 ml of CMA with 7 g of PTS as a catalyst (P 7 C 10 ), respectively.

Preparation of Mesocarbon Microbeads.
e 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.

Graphitization of CTPs.
e graphitization experiment was carried out in a graphitizing furnace.First, the samples were carbonized at 900 °C.
en, 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.e resultant graphitized products of B 0 C 0 , B 7 C 10 , and P 7 C 10 were denoted as B 0 C 0 -2400, B 7 C 10 -2400, and P 7 C 10 -2400, respectively.
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 .ermogravimetric analysis (TG-DTG) was performed on a Mettler-Toledo 851 e thermal analyzer under N 2 atmosphere with a heating rate of 10 °C•min −1 .
e 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 .e spectral excitation was provided by an Ar ion laser, using the 514.5 nm line and with proper power density on the sample surface.

Characteristics of the Parent CTP and Two CMA-Modified
CTPs.
e main characteristics of the parent CTP (B 0 C 0 ) and two CMA-modified CTPs (B 7 C 10 and P 7 C 10 ) are listed in Table 1.It can be observed that B 7 C 10 and P 7 C 10 have lower softening points, TI content, and C/H ratio than those of B 0 C 0 .While the CV and QI content of B 7 C 10 and P 7 C 10 are higher than that of B 0 C 0 .e highest CV can be obtained for B 7 C 10. So, after the modification of CTP with CMA in the presence of acid, the CV of CTP is indeed improved.

FT-IR Analysis.
FT-IR spectra of B 0 C 0 , B 7 C 10 , and P 7 C 10 are shown in Figure 1. e attribution of the peaks of B 0 C 0 is described in references [15][16][17].For the FT-IR spectra of B 7 C 10 and P 7 C 10 , 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 B 7 C 10 , 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 BO 4 [19].In the spectrum of P 7 C 10 , the characteristic peaks of PTS are at 1226 cm −1 , 1180 cm −1 , 1035 cm −1 , and 567 cm −1 [20].e appearance of the peaks indicates the presence of the matter.

TG-DTG Analysis.
e TG curves of B 0 C 0 , B 7 C 10 and P 7 C 10 are shown in Figure 2. e 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].e carbonization yield of B 0 C 0 , B 7 C 10 , and P 7 C 10 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.
e physical and chemical changes during the CTP pyrolysis can be well understood by the combination

2
Journal of Spectroscopy analysis of TG and DTG results.e DTG curves of the parent CTP and two CMA-modi ed CTPs are shown in Figure 3. e DTG curve of B 0 C 0 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 B 7 C 10 and P 7 C 10 , 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 B 7 C 10 , 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 B 7 C 10 and P 7 C 10 than in B 0 C 0 .

Optical Texture of Resultant
Semicokes.e 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].
e optical structure of the product prepared from B 0 C 0 , B 7 C 10 , and P 7 C 10 after heating at 400 °C for 1 hour is shown in Figure 4.
e optical micrograph of the parent CTP (Figure 4(a)) shows that some small mesophase spheres are produced.ese spheres have a maximum diameter of about 5 um and are very dispersed.Figure 4(b) shows the mesophase spheres obtained from B 7 C 10 after heating at 400 °C for 1 h.Compared to B 0 C 0 (Figure 4(a)), the number of mesophase spheres increases and the shape is more regular.
ese spheres have a maximum diameter of 40 um.e optical structure of P 7 C 10 treated under the same conditions is shown in Figure 4(c).Compared to B 0 C 0 (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 modi cation of CTP by CMA.But the B 7 C 10 can get larger sizes and more regular spheres.

XRD Analysis.
To study the e ect of boric acid and PTS on the graphitization process of CMA-modi ed coal tar pitch, B 0 C 0 -2400, B 7 C 10 -2400, and P 7 C 10 -2400, which are the graphitized products of B 0 C 0 , B 7 C 10 , and P 7 C 10 heated at 2400 °C and held for 2 h, respectively, were prepared and characterized by XRD and Raman spectra.
Figures 5(a)-5(c) show the XRD patterns of the three CTPs and their corresponding graphitized products.Figure 5(d) is the magni ed XRD patterns of B 0 C 0 -2400, B 7 C 10 -2400, and P 7 C 10 -2400.By comparing the XRD patterns of CTP with its corresponding graphitized products (Figures 5(a)-5(c)), it can be observed that the di raction peak centered at 25 °, which is generally indexed to (002) di raction of graphite [24], becomes sharp after graphitization while the other di raction peaks of graphite loomed.Otherwise, in the XRD patterns of B 7 C 10 (Figure 5 peaks centered at 14.67 °and 28.1 °are the di raction peaks of boric acid [25].To see clearly the small peaks of the graphitized products of B 0 C 0 -2400, B 7 C 10 -2400, and P 7 C 10 -2400, the XRD patterns are magni ed (Figure 5(d)).It is worth noting that (201) and (114) crystal planes of graphite can be obverted in the XRD patterns of B 7 C 10 -2400 but cannot be found in the XRD patterns of B 0 C 0 -2400 and P 7 C 10 -2400, which indicates B 7 C 10 -2400 possesses a higher degree of graphitization than B 0 C 0 -2400 and P 7 C 10 -2400.
According to the XRD patterns, the crystal structure parameters (d 002 , the interlayer spacing; L c , the crystallite sizes along the c-axis; G, the graphitization degree) of B 0 C 0 -2400, B 7 C 10 -2400, and P 7 C 10 -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 d 002 and L c values of B 7 C 10 -2400 are smaller than those of B 0 C 0 -2400, and the graphitization degree of B 7 C 10 -2400 is larger than that of B 0 C 0 -2400.is indicates  the graphitization degree of CMA-modi ed CTP can be increased obviously with boric acid as a catalyst.For P 7 C 10 -2400, the value of d 002 is larger than that of B 0 C 0 -2400, but the values of L c and G are smaller than those of B 0 C 0 -2400.
is shows that the graphitization degree of CMA-modi ed CTP with PTS as a catalyst cannot be increased although the carbon yield is increased.

Raman Analysis.
e 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).e I G /I D intensity ratio is considered to be an indicator of the degree of sample disorder.e larger the I G /I D value, the lower the degree of disorder [27].
Figure 6 shows the Raman spectra of B 0 C 0 -2400, B 7 C 10 -2400, and P 7 C 10 -2400.e two peaks of P 7 C 10 -2400 is little di erent from that of B 0 C 0 -2400.But for B 7 C 10 -2400, the two peaks shift to higher frequency and the intensity increases strongly.
Table 3 lists the I G /I D value of Raman spectra of B 0 C 0 -2400, B 7 C 10 -2400, and P 7 C 10 -2400.It is obvious that B 7 C 10 -2400 has higher I G /I D value than B 0 C 0 -2400 and P 7 C 10 -2400, which indicates B 7 C 10 -2400 has a higher degree of graphitization.So, the result of the Raman spectroscopy is consistent with the XRD.
From the XRD and Raman spectroscopy analysis, it can be concluded that B 7 C 10 has a higher graphitization degree than P 7 C 10 .e reason may be due to the good compatibility between boron atoms and carbon atoms.e covalent radius of boron atoms and carbon atoms is 0.088 nm and 0.077 nm, respectively.In addition, the di usion coe cient of boron atoms in the direction of the graphite crystal is as high as 6320 cm 2 /S −1 [28].erefore, it is possible that boron atoms may occupy the disordered carbon structure by the di usion of solid solution, and the defects of the disordered structure are eliminated [29].So, the modi ed CTP with boric acid as a catalyst has a higher graphitization degree than that with p-toluene sulfonic acid as a catalyst.

Conclusions
e e ect of boric acid and p-toluene sulfonic acid on the pyrolysis and graphitization process of CMA-modi ed CTP was compared.e results show that larger size and more regular mesophase spheres can be obtained from B 7 C 10 compared with P 7 C 10 under the same experimental condition.
Furthermore, the product of B 7 C 10 graphitized at 2400 °C possesses a higher graphitization degree than that of P 7 C 10 .erefore, boric acid is better than p-toluene sulfonic acid as a catalyst in the modi cation of CTP.
Data Availability e data used to support the ndings of this study are available from the corresponding author upon request.

Figure 5 :
Figure 5: XRD patterns of CTPs and their graphitized products.

Table 1 :
Physical properties of the parent CTP and two CMAmodified CTPs.