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

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. .e 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. .e characteristics of the obtained porous carbons were characterized by N2 adsorption-desorption isotherm..e results show that EDTA-Na2Mg•4H2O has a great influence on the pyrolysis andmorphology of CTP..e 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. .e 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 574.18m g and the average pore width is 4.53 nm.

Various carbon materials were prepared from different raw materials and by different methods. Hierarchical porous carbon with a large surface area of 3188 m 2 g −1 and a pore volume of 3.2 cm 3 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 ZnCl 2 activation method [17]. It has a well-developed hierarchical porous structure and its specific surface area is up to 2589 m 2 g −1 . Nitrogen-doped hierarchical porous carbon was prepared from polyaniline/silica self-aggregates, and its specific surface area is 899 m 2 g −1 [18]. A hierarchically porous carbon was prepared via a facile hardtemplating method using silica as a template, which is thoroughly removed by an in situ polytetrafluoroethylene pyrolytic process without further treatment. e specific surface area can reach up to 664 m 2 g −1 [19]. Cattail woolderived 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-Na 2 Mg•4H 2 O, which decomposes CO 2 , Na 2 CO 3 . and MgO at high temperatures and the products can be used as activators and templates for porous carbon materials. e pyrolysis and morphology behavior of the mixture of EDTA-Na 2 Mg•4H 2 O and coal tar pitch (EDTA-Na 2 Mg•4H 2 O@ CTP) was studied, and the obtained porous carbon was characterized.

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. e EDTA-Na 2 Mg•4H 2 O used in the experiment was of analytical grade and provided by Nanjing Duly Biotech Co. Ltd. (Nanjing, China).

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

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 o C min −1 and kept at this temperature for 1 hour. e N 2 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. e resulting products are denoted as MM-8-1-t, where t stands for the heat treatment temperature.
e 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. e as-obtained porous carbon is nominated as PC-8-1-800.

Measurements.
ermogravimetry and differential scanning calorimetry analysis (TG-DSC) was performed on a thermal analyzer (model Pyris Diamond, Perkin-Elmer, USA) under an N 2 atmosphere with a heating rate of 10°C min −1 . Fourier transform infrared (FT-IR) spectroscopy was acquired on a ermo 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 �1.5406Å) radiation at 40 kV and 35 mA. e morphologies of the samples were examined using TESCAN VEGA II scanning electron microscopy (SEM). e N 2 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. 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. e 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]. e 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. e peak at about 60°C, which is not accompanied by weight loss, indicates the state of CTP transformed from brittle to viscoelasticity [24]. e 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].

TG-DSC Analysis.
For MM-8-1, the TG-DSC curve becomes complicated. e 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-Na 2 Mg•4H 2 O. Further, the TG-DSC curve becomes complicated in the temperature range of 200-450°C. ere 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.

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. e 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-Na 2 Mg•4H 2 O. 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   C-H bending vibration of methyl and methylene 700-900 Aromatic out-of-plane C-H vibration with different substitution patterns 750 Out-of-plane bending vibration with four adjacent hydrogens on aromatic rings [26] Journal of Spectroscopy 3 temperature. e 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

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-Na 2 Mg•4H 2 O and carbon. With the increase of temperature, the decomposition reaction of EDTA-Na 2 Mg•4H 2 O occurs. When the temperature is higher than 600°C, all the samples only show the peaks of Na 2 CO 3 , 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-Na 2 Mg•4H 2 O has the following reactions with the increase of temperature.

SEM Analysis.
In order to study the influence of EDTA-Na 2 Mg•4H 2 O 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. e SEM image of the products is shown in Figure 4. It can be seen from the figure that when the ratio of EDTA-Na 2 Mg•4H 2 O 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-Na 2 Mg•4H 2 O 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-Na 2 Mg•4H 2 O content, the Na 2 CO 3 and CO 2 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. 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 Na 2 CO 3 and MgO have been  Figure 7. It can be seen that they are typical type IV isotherm and have strong N 2 adsorption under low pressure (P/P 0 ≤ 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. e 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. e pore structure parameters of all the samples are summarized in Table 2.

Characteristics of PC-8-1-700.
e specific surface area (S BET ) of EDTA-Na 2 Mg•4H 2 O-700 and PC-8-1-700 is 436.95 and 574.18 m 2 g −1 , while the total pore volume (V t ) is 0.25 and 0.28 cm g −1 , and the micropore volume (V mic ) is 0.13 and 0.19 cm g −1 , respectively. e increase of the surface area and pore volume may be caused by the complicated physical changes between EDTA-Na 2 Mg•4H 2 O and coal pitch at high temperature and the decomposition of EDTA-Na 2 Mg•4H 2 O at high temperature to form magnesium oxide, which is washed away by hydrochloric acid.

Conclusions
EDTA-Na 2 Mg•4H 2 O@CTP decomposes MgO, Na 2 CO 3 , and CO 2 at 600°C and the thermal reduction of MgO and C occurs at about 858°C. For the physical and chemical changes of EDTA-Na 2 Mg•4H 2 O, 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 m 2 g −1 and the average pore diameter is 4.53 nm.

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

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.  Journal of Spectroscopy 7