Pyrolysis and Morphology Behavior of EDTA-Na<sub>2</sub>Mg•4H<sub>2</sub>O and Coal Tar Pitch and Its Application for Porous Carbon

Zhang, Wen-Juan; Song, Shi-Hua; Tian, Wen-Hong

doi:https://doi.org/10.1155/2021/6684311

Journal of Spectroscopy

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Abstract Introduction Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2021 | Article ID 6684311 | https://doi.org/10.1155/2021/6684311

Pyrolysis and Morphology Behavior of EDTA-Na₂Mg•4H₂O and Coal Tar Pitch and Its Application for Porous Carbon

Wen-Juan Zhang,¹Shi-Hua Song,¹and Wen-Hong Tian²

Academic Editor: Carlos Palacio

Received22 Oct 2020

Revised22 Apr 2021

Accepted05 May 2021

Published12 May 2021

Abstract

Coal tar pitch (CTP) is a quite promising candidate for the production of porous carbons. Traditionally, the porous carbons are prepared by the heat treatment of carbon precursors in the presence of template and activator. In this paper, EDTA-Na₂Mg•4H₂O and CTP were mixed to produce porous carbons in the absence of template and activator, which were generated in situ by the heat treatment of EDTA-Na₂Mg•4H₂O. The pyrolysis and morphology behavior of the mixture of EDTA-Na₂Mg•4H₂O and coal tar pitch (EDTA-Na₂Mg•4H₂O@CTP) were studied by thermogravimetry and differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. The characteristics of the obtained porous carbons were characterized by N₂ adsorption-desorption isotherm. The results show that EDTA-Na₂Mg•4H₂O has a great influence on the pyrolysis and morphology of CTP. The pyrolysis behavior of CTP becomes complicated after the addition of EDTA-Na₂Mg•4H₂O for the physical and chemical changes of EDTA-Na₂Mg•4H₂O during the heat treatment. EDTA-Na₂Mg•4H₂O@CTP dehydrates at 160°C and decomposes Na₂CO₃ and MgO at 600°C. The surface morphology of EDTA-Na₂Mg•4H₂O@CTP changes with the EDTA-Na₂Mg•4H₂O content and heat treatment temperature. After acid washing of the product of EDTA-Na₂Mg•4H₂O@CTP heat-treated at 700°C, the obtained porous carbon material consists of micropores and mesopores. Its specific surface area is 574.18 m² g⁻¹ and the average pore width is 4.53 nm.

1. Introduction

Carbon materials are promising candidates for supercapacitor because of their chemical stability, low cost, fine conductivity, and versatile existing forms [1]. Porous carbon, one of the most important carbon materials, is of great importance for its high specific capacitance [2–5]. Various methods were adopted to synthesize porous carbons, such as chemical activation, physical activation, the combination of physical and chemical activation processes [6–8], and newly developed template-synthesized mesoporous/microporous carbons [9–15].

Various carbon materials were prepared from different raw materials and by different methods. Hierarchical porous carbon with a large surface area of 3188 m² g⁻¹ and a pore volume of 3.2 cm³ g⁻¹ was prepared by the sol-gel process of lotus seed shell and sodium phytate, followed by carbonization and NaOH activation [16]. Alginate-derived porous carbon was obtained by the nano-ZnO hard template-induced ZnCl₂ activation method [17]. It has a well-developed hierarchical porous structure and its specific surface area is up to 2589 m² g⁻¹. Nitrogen-doped hierarchical porous carbon was prepared from polyaniline/silica self-aggregates, and its specific surface area is 899 m² g⁻¹ [18]. A hierarchically porous carbon was prepared via a facile hard-templating method using silica as a template, which is thoroughly removed by an in situ polytetrafluoroethylene pyrolytic process without further treatment. The specific surface area can reach up to 664 m² g⁻¹ [19]. Cattail wool-derived porous carbon was fabricated by using calcium carbonate as a hard template and potassium oxalate monohydrate as activation [20]. High-quality meso/micropore-controlled hierarchical porous carbon was synthesized by a hard template method utilizing rice husk biochar and then used to adsorb copper ions from an aqueous solution [21].

Coal tar pitch (CTP) is a kind of complex material consisting of aromatic compounds with a broad molecular weight distribution [22]. It is a quite promising candidate for the production of porous carbons for its low cost, abundant resources, and the ability to generate graphitizable carbons [23]. In this paper, CTP was used as a carbon material precursor to produce porous carbons in the presence of EDTA-Na₂Mg•4H₂O, which decomposes CO₂, Na₂CO₃. and MgO at high temperatures and the products can be used as activators and templates for porous carbon materials. The pyrolysis and morphology behavior of the mixture of EDTA-Na₂Mg•4H₂O and coal tar pitch (EDTA-Na₂Mg•4H₂O@CTP) was studied, and the obtained porous carbon was characterized.

2. Experiment

2.1. Materials

CTP, the softening point of which is 108°C, was purchased from the Wuhan Iron and Steel Co. Ltd. (Wuhan, China) and its toluene insoluble and quinoline insoluble content were 43% and 6.8%, respectively. The EDTA-Na₂Mg•4H₂O used in the experiment was of analytical grade and provided by Nanjing Duly Biotech Co. Ltd. (Nanjing, China).

2.2. Preparation of Materials

For a typical run, CTP with a particle size of below 100 um and EDTA-Na₂Mg•4H₂O were used as raw materials. Their mixture process was carried out in a traditional ball mill at a rotation rate of 450 rpm for 2 h. The obtained mixture is named as MM-a-b, where a is the mass of EDTA-Na₂Mg•4H₂O and b is the mass of CTP.

2.3. Heat Treatment and Acid Cleaning Treatment

Heat treatment of the MM-8-1 was carried out in a tube furnace. About 5 g sample was heated to a designated temperature at a rate of 4 ^oC min⁻¹ and kept at this temperature for 1 hour. The N₂ stream was introduced into the tube furnace throughout the carbonization. In this paper, the samples were heated to 500°C, 600°C, 700°C, and 800°C and cooled to room temperature. The resulting products are denoted as MM-8-1-t, where t stands for the heat treatment temperature.

The MM-8-1-800 was then washed with HCl solution (6 mol L⁻¹) and stirred for 4 h. Finally, it was filtered and washed thoroughly with distilled water and ethanol to remove the residual ions and byproducts and then dried in an oven at 110°C. The as-obtained porous carbon is nominated as PC-8-1-800.

2.4. Measurements

Thermogravimetry and differential scanning calorimetry analysis (TG-DSC) was performed on a thermal analyzer (model Pyris Diamond, Perkin-Elmer, USA) under an N₂ atmosphere with a heating rate of 10°C min⁻¹. Fourier transform infrared (FT-IR) spectroscopy was acquired on a Thermo Nicolet-360 FT-IR spectrometer in the range of 4000–400 cm⁻¹. X-ray diffraction (XRD) was carried out on the PANalytical X’Pert PRO X-ray diffraction instrument with CuKα (λ = 1.5406 Å) radiation at 40 kV and 35 mA. The morphologies of the samples were examined using TESCAN VEGA II scanning electron microscopy (SEM). The N₂ adsorption/desorption isotherms at −196°C were measured using a Micromeritics TriStar 3020 porosimeter. Specific surface area (SSA) was calculated by the Brunauer–Emmett–Teller (BET) method and pore size distributions were analyzed by the Barrett–Joyner–Halenda (BJH) method.

3. Results and Discussion

3.1. TG-DSC Analysis

Figure 1 shows the TG-DSC curves of parent CTP and MM-8-1. It can be seen from the TG-DSC curve of parent CTP that it decomposes in one single mass loss stage in the temperature range of 25–900°C. The weight loss is mainly due to the removal of gases and light compounds generated via thermal polymerization and the cracking of side chains of aromatic rings [22]. The DSC profile of parent CTP is characterized by a small endothermic peak at about 60°C, two exothermic peaks close together at about 530°C, and a big exothermic peak at 640°C. The peak at about 60°C, which is not accompanied by weight loss, indicates the state of CTP transformed from brittle to viscoelasticity [24]. The three exothermic peaks at about 530°C and 640°C, which are associated with weight loss during the heat treatment process, are corresponding to the reactions of thermal polymerization taken place at these temperatures [24].

For MM-8-1, the TG-DSC curve becomes complicated. The slope of the TG curve becomes steep in the temperature range of 140–200°C and the corresponding DSC curve at this stage has an endothermic peak, which corresponds to the removal of crystal water from EDTA-Na₂Mg•4H₂O. Further, the TG-DSC curve becomes complicated in the temperature range of 200–450°C. There is a large exothermic peak at 408°C, and the thermal weight loss rate at this temperature is the largest, indicating that violent and complex physical and chemical reactions occur at this stage. At last, in the temperature range of 450–900°C, the DSC curve shows two endothermic peaks at 500°C and 858°C and two exothermic peaks at 564°C and 805°C, indicating that physical or chemical reaction occurs in the system at these temperatures.

3.2. FT-IR Analysis

To explore the structural changes of MM-8-1 during the pyrolysis, the raw materials and the products of MM-8-1 heat-treated at 500°C, 600°C, 700°C, and 800°C are characterized by FT-IR, which are illustrated in Figure 2. The attribution of the peaks in FT-IR spectra is listed in Table 1. From Figure 2, it can be observed the FT-IR spectrum of MM-8-1 is the superposition of CTP and EDTA-Na₂Mg•4H₂O. During the pyrolysis of MM-8-1, the peak at about 3345 cm⁻¹, which is attributed to O–H stretching vibration, disappears when it is heated to a temperature higher than 500°C. Otherwise, the peak at 750 cm⁻¹, which corresponds to the out-of-plane bending vibration with four adjacent hydrogens on aromatic rings, becomes small with the increase of heat treatment temperature. The peak at about 1600 cm⁻¹ disappears after it is heated to a temperature higher than 500°C, and the intensity of the peak at 1420 cm⁻¹ enhances with the increasing temperature. Further, there is little difference for the FT-IR spectra of MM-8-1-500, MM-8-1-600, MM-8-1-700, and MM-8-1-800.

3.3. XRD Analysis

In order to further determine the physical and chemical changes during the heat treatment of MM-8-1, all the samples were characterized by XRD, as shown in Figure 3. It can be seen that there are many peaks in the raw material of MM-8-1, including XRD diffraction peaks of EDTA-Na₂Mg•4H₂O and carbon. With the increase of temperature, the decomposition reaction of EDTA-Na₂Mg•4H₂O occurs. When the temperature is higher than 600°C, all the samples only show the peaks of Na₂CO₃, MgO, and C. In addition, with the increase of temperature, the intensity of diffraction peak further increased, indicating that the crystallinity of corresponding products increased. Combined with the results of TG-DSC, it can be concluded that EDTA-Na₂Mg•4H₂O has the following reactions with the increase of temperature.

3.4. SEM Analysis

In order to study the influence of EDTA-Na₂Mg•4H₂O content on the surface morphology of CTP, MM-4-1, MM-6-1, MM-8-1, and MM-10-1 were heated to 600°C and kept for 1 hour. The SEM image of the products is shown in Figure 4. It can be seen from the figure that when the ratio of EDTA-Na₂Mg•4H₂O to CTP is 4 : 1, the surface morphology is like a layered petal (see Figure 4(a)). As the ratio increases, filaments are formed in the system (see Figure 4(b)). When the ratio of EDTA-Na₂Mg•4H₂O to CTP is 8 : 1, the surface morphology is a relatively regular block, and the size of the block is about 2.5 um (see Figure 4(c)). When the concentration continues to increase, the blocks disappear, forming a relatively dense uneven surface (see Figure 4(d)). It can be concluded that, with the increase of EDTA-Na₂Mg•4H₂O content, the Na₂CO₃ and CO₂ in the pyrolysis process of the system continue to increase, and the activation effect on the system is continuously enhanced and leads to the change of the morphology.

Figure 5 shows the surface morphology of parent CTP and MM-8-1 heat-treated at 500°C, 600°C, 700°C, and 800°C. It can be observed the surfaces of parent CTP change greatly with the increase of temperature. When it is heated to 500°C, the surface becomes relatively neat (see Figure 5(a)). However, the surface becomes messy when it is heated to 600°C and 700°C (see Figures 5(c) and 5(d)), which may be caused by the reaction of thermal polymerization taken place at about 530°C. In addition, some pores exist on the surface of parent-700 (see Figure 5(c)). When it is heated to 800°C, the surface is arranged in a relatively orderly manner.

For MM-8-1, the surface morphology is different from that of parent CTP at the same heat treatment temperature. When it is heated to 500°C and 600°C, some rock sugar lumps with a particle size of about 1–3 μm are generated (see Figures 5(e) and 5(f)). When the temperature is higher than 600°C, the resulting material appears fluffy (see Figures 5(g) and 5(h)). It can be inferred that as the temperature increases, the reaction speed increases, leading to the activated substances in the system increase, and the morphology changes.

3.5. Characteristics of PC-8-1-700

Figure 6 shows the XRD patterns of MM-8-1-700 before and after acid washing. It can be seen that only the diffraction peak of C exists after washing, indicating that Na₂CO₃ and MgO have been thoroughly cleaned. The chemical reaction equations involved are shown as follows:

The N₂ adsorption and desorption isotherms of PC-8-1-700 and EDTA-Na₂Mg•4H₂O-700 are shown in Figure 7. It can be seen that they are typical type IV isotherm and have strong N₂ adsorption under low pressure (P/P₀ ≤ 0.35), indicating that the materials have a strong force with nitrogen. With the increase of relative pressure, the adsorption and desorption capacity of nitrogen are obviously different. The adsorption and desorption curves cannot coincide and hysteresis loops appear, which indicates that there are a lot of micropores and mesopores in the materials. Meanwhile, the pore size distribution in Figure 7(b) also proves the coexistence of micropores and mesopores in the materials. The pore structure parameters of all the samples are summarized in Table 2. The specific surface area (S_BET) of EDTA-Na₂Mg•4H₂O-700 and PC-8-1-700 is 436.95 and 574.18 m² g⁻¹, while the total pore volume (V_t) is 0.25 and 0.28 cm³ g⁻¹, and the micropore volume (V_mic) is 0.13 and 0.19 cm³ g⁻¹, respectively. The increase of the surface area and pore volume may be caused by the complicated physical changes between EDTA-Na₂Mg•4H₂O and coal pitch at high temperature and the decomposition of EDTA-Na₂Mg•4H₂O at high temperature to form magnesium oxide, which is washed away by hydrochloric acid.

(a)

(b)

4. Conclusions

EDTA-Na₂Mg•4H₂O@CTP decomposes MgO, Na₂CO₃, and CO₂ at 600°C and the thermal reduction of MgO and C occurs at about 858°C. For the physical and chemical changes of EDTA-Na₂Mg•4H₂O, it has a great influence on the pyrolysis and morphology of CTP. After acid washing of the carbon material heat-treated at 700°C, the obtained porous carbon material consists of micropores and mesopores. Its specific surface area is 574.18 m² g⁻¹ and the average pore diameter is 4.53 nm.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This research was supported by the Science and Technology Project of Department of Education of Jiangxi Province (GJJ170948).

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Copyright © 2021 Wen-Juan Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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