Improvement of Activated Carbon from Jixi Bituminous Coal by Air

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Improvement of Activated Carbon from Jixi Bituminous Coal by Air Preoxidation Dongdong Liu, Jihui Gao,* Qingxi Cao, Shaohua Wu, and Yukun Qin School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin, Heilongjiang 150001, People’s Republic of China ABSTRACT: Air preoxidation has been demonstrated as a good method to change the structure of Jixi bituminous char for controlling development of porosity at low burnoff under physical activation. In this work, the influences of different oxidation times in air at 200 °C (0, 3, 15, 48, and 96 h) were studied. The removal of aliphatic chains and the increase of the type and quantity of oxygen functional groups occur at the preoxidation stage, resulting in an open structure for oxidized coals; however, slight improvement is made on the porosity and surface morphology. In the pyrolysis stage, the evolution of different oxygencontaining structures plays a more important role than the loss of plastic behavior on disordered conversion of oxidized chars, leading to production of more active sites and development of the meso- and macropores. In the activation stage, serious carbon losses at the surfaces of particles is observed in JX-800-59.1 with the low values of SBET (589.41 m2/g) because of the unfavorable diffusion of activated gas. Alternatively, the well-developed pore structure of JXO15-800 and JXO48-800 chars promotes the favorable diffusion of activated gas, thereby hindering the severe carbon losses at the particle surface, but JXO48-800 char with more disordered structure and active sites has a high value of SBET (818.98 m2/g) at a low value of burnoff of 30.5% compared to the values of SBET (705.69 m2/g) of JXO15-800 char at a value of burnoff of 42.6%. approximately 50−80% during activation.16−18 However, at this burnoff value, the surface of the particles has been damaged severely and the external loss of quality only promotes the total amount of carbon loss but does not produce the pores that increase the specific surface area. Zhu et al.19,20 adjusted the activation temperature from 800 to 950 °C and the type and dosage of activating gases (steam and CO2). They concluded that the change of the activation condition is unable to effectively control an excessive loss of the particle surface. A specific analysis is as follows: the diffusion of the activated gas (such as steam or CO2) and activated product (such as H2 and CO) is hindered by the coal char structure produced by pyrolysis that has a large amount of ordered microcrystalline structure and fewer pores and active sites, which are related to the presence of plastic properties of bituminous coal during pyrolysis.16,17,21−24 He et al.25 found that the higher amount of transferable hydrogen from the decomposition of aliphatic hydrocarbons stabilizes other radical groups (such as •CnH2n + 1, •OCnH2n + 1, and substituted benzene) at 400−600 °C to produce the metaplast material, which plugs the pores and acts as a lubricant between aromatic layers to promote the ordered conversion of the microcrystalline structure. Therefore, an ideal char structure, which includes a well-developed pore structure, a large amount of disordered crystalline structure, and active sites obtained by pyrolysis, has been one of the most important factors in controlling the pore development at a low burnoff value during activation. In comparison to other pretreatment methods for preparation of AC, such as the template method and solvent extraction,26−29 air preoxidation provides an efficient method to change the coal

1. INTRODUCTION The main gas pollutants (e.g., SO2, NOx, and CO2) from a large coal-fired power plant constitute the major cause of pollution in the atmosphere, which not only threatens human health but also prevents economic development. In the case of the current technologies in gas desulfurization, they must all be based on economic sustainability and environmental sustainability to satisfy requirements of environmental protection.1−4 For the removal of sulfur dioxide, the traditional wet flue gas desulfurization technology using calcium-based absorbents now faces the issues of its high cost, large water consumption, and release of CO2. Alternatively, carbon-based porous solid adsorbents, such as activated carbon (AC), are a promising choice to achieve the adsorption and catalytic conversion of SO2 because of their low price, low water use, and ability to generate byproducts (such as sulfuric acid).5−9 In the case of AC, the hierarchical structure (micro- and mesopores) is critical to the SO2 removal process. Sun et al.10 and Shu et al.11 reported that the adsorption and catalysis processes of SO2 are performed within the micropores, and the developed mesopores promote the migration and storage of produced sulfuric acid. In addition, a high specific surface area (SBET), which is related to the degree of well-developed pores, promotes desulfurization.12,13 In the special case of coal-fired power plant desulfurization, coal is the most suitable raw material for AC rather than biomasses and some wastes. In addition, the traditional physical activation with steam or CO2 resulting from high-temperature flue gas of coal-fired boilers is preferred for the production of coal-based AC over chemical activation because of its low cost and environmental friendliness.14,15 In previous significant studies of traditional physical activation, most of the works reported that an obvious hierarchical structure and specific surface area (SBET) values between 500 and 800 m2/g of bituminous coal-based AC are formed for high burnoff values of © XXXX American Chemical Society

Received: November 1, 2016 Revised: January 12, 2017 Published: January 18, 2017 A

DOI: 10.1021/acs.energyfuels.6b02875 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Proximate and Ultimate Analyses of the Acid-Treated Sample proximate analysis (wt %)

a

ultimate analysis (wt %, daf)

sample

Vdaf

FCdaf

Ad

Mad

Cdaf

Hdaf

Odafa

Ndaf

Sdaf

JX

39.66

60.34

0.125

3.62

74.81

19.49

4.01

1.31

0.38

By difference.

Figure 1. Schematic figure of the tube furnace reactor system.

char structure because of the low expense and simplicity.30−32 Because of the high reactivity of air (oxygen), it is relatively easy to effectively control the chemical reaction between air and coal at a moderate temperature during the preoxidation stage, which promotes pore development and microcrystalline adjustment at the subsequent pyrolysis and activation stage. Previously, de la Puente et al.33 used thermogravimetric analysis (TGA) to study the pyrolysis behavior of bituminous coal and its oxidized coal. They concluded that the plastic behavior of oxidized coal could be effectively suppressed by air preoxidation via removal of the aliphatic CH groups. Teng et al.16,17 showed that the preoxidation method is favorable to the loss of the plastic properties of bituminous coal and the growth of char microporosity. The air preoxidation process presented in this paper is relatively similar to the oxidation step of the cyclic oxygen chemisorption−desorption method, which has been studied deeply by Francisco et al.34−38 They concluded that evolution of oxygen functional groups promotes pore development, thereby enabling AC with a high SBET/burnoff ratio even after several cycles, and that the mesopores are first produced and then micropores are formed with the increase of the cycle number. Although the effect of air preoxidation in the preparation process of coal-based AC on the structure of coal chars plays an important role in controlling the final AC product, this effect has rarely been studied in detail. The goal of this paper to prepare AC with high SBET at a relatively low burnoff via an air preoxidation treatment. The effects of different oxidation times at 200 °C in air on physical and chemical structures of samples at the preoxidation and pyrolysis stages are studied, respectively. In comparison of the development of the pore structure at different burnoffs for different chars with typical characteristics in the activation stage, the relationship between structural properties of chars and pore structure of the final AC product at different external loss of quality was determined in more detail. The feature parameters of all samples were obtained by nitrogen adsorption, scanning

electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Jixi bituminous coal from China was used in this experiment and was crushed and sieved to a particle size of 250−380 μm. To eliminate the interference of ash in coal, the sample coal was processed using 6 mol/L HCl and 40 wt % HF sequentially in accordance with the demineralization procedure.39 First, the mixture including 4 g of raw coal and 200 mL of 6 mol/L HCl was stirred for 15 h, which was then filtered and added to 150 mL of 40 wt % HF, followed by stirring again for 15 h. After filtration, the chloride ions in the mixture were washed repeatedly using deionized water and then the hot-air treatment at 80 °C was applied for 12 h to remove the water in demineralized coal before use.40 After the demineralization procedure, the proximate and ultimate analysis data of acid-treated samples were obtained, as shown in Table 1. The content of the ash in the acid-treated sample was below 1%, and this sample was denoted as JX. In addition, JX was oxidized in air at 200 °C for 3, 15, 48, and 96 h, and these samples are denoted as JXO3, JXO15, JXO48, and JXO96, respectively. 2.2. Experimental Process. (1) Pyrolysis experiment: 3 g of the sample coals (JX, JXO3, JXO15, JXO48, and JXO96) were placed in a three-stage fixed-bed reactor, heated at 8 °C/min to 800 °C, and then maintained for 60 min in a N2 flow of 450 mL/min, as illustrated in Figure 1. Next, the sample coals JX, JXO3, JXO15, JXO48, and JXO96 were rapidly cooled under a N2 atmosphere and denoted as JX-800, JXO3-800, JXO15-800, JXO48-800, and JXO96-800, respectively. (2) Activation experiment: these char samples were continuously heated to 900 °C under a N2 atmosphere, then were held for the appropriate time under CO2 to obtain AC with different porous structures at different burnoffs, and were denoted as (JX-800, JXO3-800, JXO15-800, JXO48800, and JXO96-800)- burnoff values. 2.3. Measurement Analysis. The structural parameters of the crystalline structure were obtained by a D/max-rb X-ray diffractometer, with the measurement performed in the 2θ range from 5° to 85° at a scan rate of 3°/min. Next, the char powder was investigated by Raman spectroscopy using a 532 nm wavelength laser and observing the scatter in the range of 1000−1800 cm−1. SEM (Quanta 200) was performed using an instrument operated at 200 kV to visualize the surface topography of samples. These samples were analyzed by Fourier B


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Energy & Fuels transform infrared spectroscopy (FTIR) in the 500−4000 cm−1 range, employing standard pellets of the KBr matrix, and 200 scans were performed at a resolution of 4 cm−1. The pore structure characteristics of prepared samples were obtained by nitrogen adsorption at an analysis temperature of 77 K using a Micromeritics adsorption apparatus (ASAP2020), with the measurement performed for the adsorption data in the relative pressure (P/P0) range from 10−7 to 1.41 The prepared samples were degassed under vacuum at 473 K for 12 h before N2 adsorption analysis. The specific surface area (SBET), the micropore area (Smic), the micropore volume (Vmic), and pore size distribution were calculated using the Brunauer−Emmett−Teller (BET) equation, t-plot method, Horvath−Kawazoe (HK) method, and non-local density functional theory (NLDFT), respectively.42−44 In addition, the total pore volume (Vt) was determined at 0.98 relative pressure.

time was attributed to asymmetric CH3− and CH2−, symmetric CH2−, and asymmetric CH3−, respectively.45 The intensity of the three bands are obvious for JX, but the disappearance of 2830 and 2953 cm−1 can be observed in air at 200 °C for 3 h (JXO3); however, the aliphatic bands (1460, 2830, and 2953 cm−1) have completely disappeared above 15 h (JXO15/48/96). These changes indicate that the aliphatic structure is removed gradually with the increase of the oxidation time. Most of the works found that the loss of plastic behavior for bituminous coal during pyrolysis is derived from the removal of the aliphatic structure.16,17,21−24 In addition, the aliphatic structure in coal is in the form of cross-linking bonds (methylene groups) connecting aromatic rings and as side chains (methylene and methyl groups) attached to aromatic rings.46 Thus, the cleavage of aliphatic chains at the air preoxidation stage will likely lead to the change of the microcrystalline structure for oxidized coals and corresponding oxidized chars. Then, the changes in oxygen-containing structures in the 1800−1000 cm−1 zone in FTIR spectra are investigated further by curve-fitted bands according to the previous literature,45,47 and an example of curve-fitted FTIR spectra of the 1800−1000 cm−1 zone for JX and JXO15 is shown in Figure 3. According to the result of curve-fitted bands, some ratios of integrated absorbance areas are used to quantify the change of oxygen functional groups of unoxidized and oxidized coals during air preoxidation. The specific relationships of these parameters are the following:

3. RESULTS AND DISCUSSION 3.1. Chemical Structure Analysis by FTIR. The FTIR spectra of unoxidized and oxidized coals are shown in Figure 2.

CO/Car = 1780−1650 cm−1 zone/1605 cm−1 band (1)

COOH/Car = 1700 cm−1 zone/1605 cm−1 band

(2)

C−O/Car = 1260−1030 cm−1 zone/1605 cm−1 band (3)

Equations 1−3 give a quantitative assessment of the changes of CO (including carboxylic acids, carbonyl acids, aryl esters, etc.), COOH, and other C−O−R (including aryl ethers, etc.) structures against the aromatic structure network of samples. The results of structural parameters from this curve-fitting analysis (1800−1000 cm−1 zone) for unoxidized and oxidized coals during air preoxidation are given in Table 2. As shown in Table 2, small quantities of oxygen functional groups (CO mainly as carboxylic acids and C−O) can be found in JX but JXO3 has an increase in CO (mainly as carboxylic acids) and C−O structures, indicating the selective

Figure 2. FTIR spectra of unoxidized and oxidized coals.

First, the gradual disappearance of three bands at approximately 1460, 2830, and 2953 cm−1 with the increase of the oxidation

Figure 3. Curve-fitted FTIR spectrum of the 1800−1000 cm−1 zone for (a) JX and (b) JXO15. C


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Energy & Fuels Table 2. Curve Fitting for the 1800−1000 cm−1 Region of Unoxidized and Oxidized Coals CO/Car COOH/Car C−O/Car

JX

JXO3

JXO15

JXO48

JXO96

0.26 0.26 0.72

0.60 0.56 1.05

1.27 0.76 1.39

1.86 0.91 1.92

1.90 1.02 2.01

pyrolysis, although a part of the aliphatic structure is removed and a few oxygen functional groups are produced for JXO3-800. Such surface morphology of JX and JXO3 will become a barrier for the subsequent diffusion of gas molecules during activation. Significant changes in surface morphology of oxidized chars prepared with the gradual increase of oxidation time from 15 to 96 h can be seen observed as a result of the loss of plastic properties and the evolution of oxygen functional groups by almost complete oxidation. There is no metasplast material available to block the pores because of the complete removal of the aliphatic structure; however, the generation of more new pores may be closely related to the change of different types of oxygen functional groups during pyrolysis. According to the literature,36,49,50 oxygen functional groups with different stability may decompose and combine with the carbon structure at different pyrolysis temperatures to release a large amount of volatile matter (such as CO2 and CO) and generate active sites, resulting in the development of a wider porosity. 3.3. Pore Structure Analysis by N2 Adsorption. The N2 adsorption−desorption isotherms and the SBET and Smic values of unoxidized and oxidized coals and carbonized chars are given in Figure 5 and Table 3. The N2 isotherms of unoxidized and oxidized coals exhibit a typical type I characteristic, according to the International Union of Pure and Applied Chemistry (IUPAC) classification, without the appearance of a hysteresis loop, showing all materials possess microporous features. Next, the N2 adsorption capacities and the values of SBET and Smic of unoxidized and oxidized coals are still small, although those of oxidized coals increase gradually with the increase of the oxidation time, indicating the generation of fewer pores. However, the knee becomes broader and rounder for JXO48 and JXO96 at a relative pressure of less than 0.2, implying the presence of wider micropores under a longer oxidation time. The above results represent that there is only a slight improvement on the pore structure with the increase of the oxidation time during the air preoxidation stage. With regard to the unoxidized and oxidized chars produced by pyrolysis at 800 °C for 60 min, first, the N2 adsorption capacities and SBET and Smic values of JX-800 and JXO3-800 are also very low, indicating the generation of fewer pores. These results are related to the formation of metaplast material, which blocks the pores during the plastic stage. Then, the N2 adsorption isotherms of JXO15-800, JXO48-800, and JXO96-800 exhibit type I at low pressures and type IV at high pressures. These isotherms become steep. N2 adsorption capacities at low pressures increase

oxidation of aliphatic structures at the beginning of air preoxidation to produce the specific oxygen functional groups. The presence of new bands near 1650 cm−1 (carbonyl acids) and 1770 cm−1 (aryl esters) can be observed in JXO15, resulting in an obvious increase in CO, COOH, and C−O groups. The quantity of CO, COOH, and C−O groups continues to increase for JXO48 and JXO96, but the percentage of COOH in CO groups has an obvious decrease (namely, a rapid increase in carbonyl acids and aryl esters with high stability), indicating a progressive conversion of oxidized coals into a relatively stable structure with the increase of the oxidation time during the air preoxidation stage. In addition, there is a slow increase in parameters of oxygen functional groups from JXO48 to JXO96, indicating that longer oxidation time (96 h) may have a limited role on the further development of oxygen functional groups. Thus, the predominance of oxidation reactions, which promotes the production of more oxygen type and quantity of oxygen functional groups with the increase of the oxidation time, is ensured during the air preoxidation stage. Francisco et al.35−37,48 reported that the increase of the type and quantity of oxygen functional groups for oxidized coals can be important for pore development at a subsequent pyrolysis stage. 3.2. Surface Morphology Analysis by SEM. The SEM images of unoxidized and oxidized coals and carbonized chars are given in Figure 4. From panels a−e of Figure 4, there are no obvious differences in the surface morphology of unoxidized and oxidized coals, although a slight rough surface in oxidized coals depends upon the oxidation reaction between oxygen and carbon, suggesting the generation of more oxygen functional groups with an increasing oxidation time at the preoxidation stage, as inferred from the result of FTIR spectra analysis in Table 2. From panels f−j of Figure 4, an extremely smooth surface with almost no porosity is observed in the JX-800 char, resulting from the development of plastic behavior in the pyrolysis stage. Moreover, the surface morphology of JXO3-800 is similar to that of JX-800, indicating that the plastic behavior still exists during

Figure 4. SEM images from unoxidized and oxidized coals and carbonized chars. D


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Figure 5. N2 adsorption/desorption isotherms of (a) unoxidized and oxidized coals and (b) carbonized chars.

Table 3. Specific Surface Area (SBET) and Micropore Area (Smic) Values of Unoxidized and Oxidized Coals and Carbonized Chars SBET Smic

JX

JXO3

JXO15

JXO48

JXO96

JX-800

JXO3-800

JXO15-800

JXO48-800

JXO96-800

21 18

27 22

40 35

54 48

78 71

10 4

15 10

123 100

156 111

180 123

Figure 6. XRD of (a) unoxidized and oxidized coals and (b) carbonized chars.

Table 4. XRD Data of Unoxidized and Oxidized Coals and Carbonized Chars La (Å) Lc (Å) d002 N

JX

JXO3

JXO15

JXO48

JXO96

JX-800

JXO3-800

JXO15-800

JXO48-800

JXO96-800

23.759 14.534 3.545 4.10

23.419 14.813 3.534 4.19

21.824 15.285 3.470 4.4

20.441 15.951 3.401 4.69

20.112 16.089 3.391 4.74

24.912 13.721 3.501 3.92

25.165 13.514 3.581 3.77

26.081 13.002 3.653 3.56

27.145 12.758 3.792 3.36

27.312 12.701 3.814 3.33

behavior has advantages for developing mesopores during pyrolysis. 3.4. Crystal Structure Analysis by XRD. The XRD profiles of the unoxidized and oxidized coals and the chars prepared by pyrolysis at 800 °C for 60 min are given in Figure 6. There are two obvious broad diffraction peaks at 2θ = 24−27° and 41−44° in all samples, which are related to an interplanar spacing between the two aromatic layers and the degree of condensation of aromatic layers.51 According to the peak fitting treatment used by Liu et al.,52 XRD profiles of all samples are processed using the software Origin 9.1 to obtain some important parameters regarding the microcrystalline structure (such as the aromatic structure layer distance d002, stacking height Lc, width La, and

evidently, and a clear hysteresis loop appears gradually as the oxidization time continues to increase, leading to an obvious increase in SBET and Smic values. The development of porous structures in JXO15-800, JXO48-800, and JXO96-800 are related to the disappearance of the plastic properties and the evolution of different oxygen functional groups during pyrolysis. However, the changes in the adsorption isotherms and hysteresis loop of JXO48-800 and JXO96-800 become more obvious at a relative pressure of more than 0.1; moreover, the SBET value of JXO48800 and JXO96-800 also exhibits a distinct growth, whereas the Smic value increased slowly. This result indicates that the evolution of more oxygen functional groups obtained by a long oxidization time (48 and 96 h) rather than the loss of plastic E


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Figure 7. Raman spectra of (a) unoxidized and oxidized coals and (b) carbonized chars.

Table 5. Summary of Raman Band Assignment band name

band position (cm−1)

description

bond type

D2 G D3 D1 D4

1620 1590 1530 1350 1200

graphite E22g; carbonyl group CO graphite E22g; aromatic ring quadrant breathing; alkene CC aromatics with 3−5 rings; semi-circle breathing of aromatic rings; methylene or methyl; amorphous carbon structures C−C between aromatic rings and aromatics with no less than 6 rings; introduced disorder carbon Caromatic−Calkyl; aromatic (aliphatic) ethers; C−C on hydroaromatic rings; hexagonal diamond carbon sp3; C−H on aromatic rings

sp2 sp2 sp2 and sp3 sp2 sp2 and sp3

number of aromatic layers N = Lc/d002), which are given in Table 4. As shown in Table 4, in comparison to the parameters of JX, the values of La and d002 of oxidized coals decrease and their values of Lc and N increase gradually with the increase of the oxidation time. The aliphatic structure exists in coal as aliphatic chains to connect aromatic nuclei and as small molecule substances between the aromatic layers.46 The breaking of aliphatic chains and removal of small molecule substances during air preoxidation leads to the depolymerization of aromatic nuclei, the decrease of layer distance, and the increase of the stacking height and number of aromatic layers. However, the longer oxidization time (96 h) cannot continue to obviously improve the size of microcrystalline structures, showing that the removal of the aliphatic structure rather than the introduction of oxygen functional groups has a major impact on the parameters of microcrystalline structures during the air preoxidation stage. Alternatively, the change in parameters of JX-800 indicates that its microcrystalline structure has been changed into a graphite-like structure because of the presence of plastic behavior during pyrolysis. In comparison to the parameters of JX-800, the values of Lc and N decrease and the values of La and d002 increase for oxidized chars, indicating that the microcrystalline structure has been transformed into a type of disordered structure during pyrolysis because of the evolution of oxygen functional groups and the loss of plastic behavior during pyrolysis. The crosslinking reaction of oxygen functional groups at a low temperature (such as carboxyl groups) produces more cross-linking bonds, and no metaplast material acts as a lubricant between aromatic layers, which hinder the movement of aromatic layers toward order during pyrolysis.53 Moreover, the increase of La is related to the connecting role of cross-linking bonds between aromatic layers rather than condensation of aromatic layers.54 The vertical stacking and condensation of aromatic layers can be hindered by the existence of heterocyclic compounds containing oxygen and

oxygen functional groups (such as ether and carbonyl acids) with high stability.55−57 These results show that the evolution of a large amount of oxygen-containing structures plays a more important role than the loss of plastic behavior on the change of microcrystalline size during pyrolysis. 3.5. Carbon Structure Analysis by Raman. The Raman spectra of unoxidized and oxidized coals and the carbonized chars prepared by pyrolysis at 800 °C for 60 min are given in Figure 7. According to the peak fitting treatment using five bands by Li et al.,58 Table 5 shows the assignment of the five bands. The Raman spectra of all samples are processed with a smoothing function, baseline correction, and normalization using Origin 9.1 software to obtain the parameters of different hybrid carbon structures, as shown in Figure 8 and Table 6. Importantly, the different band area ratios represent the relative structure;59−61

Figure 8. Deconvolution of the Raman spectrum using five bands for JX. F


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Energy & Fuels Table 6. Raman Data of Unoxidized and Oxidized Coals and Carbonized Chars JXO15

JXO48

JXO96

JX-800

ID1/IG

2.436

JX

2.477

2.664

2.877

2.890

3.197

3.224

3.600

4.147

4.151

ID3/IG

0.712

0.708

0.512

0.494

0.485

1.922

2.013

2.513

3.165

3.171

ID4/IG Iall

0.425 320.56

JXO3

0.411 316.12

0.364 294.65

0.325 281.64

0.320 278.19

0.611 310.62

JXO3-800

0.675 321.51

JXO15-800

0.809 349.60

JXO48-800

0.961 376.04

JXO96-800

0.972 382.98

Figure 9. N2 adsorption isotherms and pore size distribution of JX, JXO15, and JXO48 chars during activation.

can be also related to active sites in samples. In addition, the total spectral area (Iall) can be expressed to sample reactivity under spectral normalization treatment. As shown in Table 6, in comparison to the parameters of JX, the values of ID1/IG of oxidized coals increase, whereas ID3/IG, ID4/

this is ascribed to the following: (1) ID1/IG can be related to the defect degree of the microcrystalline structure; (2) ID3/IG is considered as the amorphous carbon; and (3) ID4/IG can be described as the relative quantity of cross-linking bonds. In particular, the defect (ID1/IG) and cross-linking bonds (ID4/IG) G


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Energy & Fuels Table 7. Pore Structure Parameters of JX, JXO15, and JXO48 Chars during Activation sample

SBET (m2/g)

Vt (m3/g)

Vmic (m3/g)

non-Vmic (%)

Dap (nm)

JX-800-15.5 JX-800-25.9 JX-800-59.1 JXO15-800-26.8 JXO15-800-42.6 JXO48-800-15.9 JXO48-800-30.5

109.41 297.74 589.41 345.51 705.69 380.54 818.98

0.065 0.142 0.298 0.184 0.324 0.337 0.417

0.062 0.127 0.217 0.117 0.261 0.201 0.363

4.6 12.1 27.2 36.41 19.44 40.36 12.95

2.38 1.91 2.0 2.13 1.83 3.54 2.00

IG, and Iall decrease gradually with the increase of the oxidation time. The aromatic nuclei in coal have been depolymerized by the removal of aliphatic side chains, cross-linking bonds, and small molecule substances with high reactivity during the air preoxidation stage, leading to a decrease in the amorphous sp2 bonding carbon atoms and sp2−sp3 bonding carbon atoms. However, the introduction of a large amount of oxygen functional groups attached to aromatic rings results in the increase of isolated and defective sp2 bonding carbon atoms. The above changes in hybrid carbon structures lead to the decrease of sample reactivity during air preoxidation. Alternatively, in comparison to the parameters of JX-800, all four parameters of oxidized chars increase as the oxidation time increases. The constant generation of active sites caused by evolution of some oxygen-containing structures, which includes the breaking of oxygen functional groups with low stability and the existence of heterocyclic compounds containing oxygen and oxygen functional groups with high stability, facilitates the formation of the isolated and defective sp2 and amorphous sp2 bonding carbon atoms, finally leading to the high reactivity of oxidized chars. These results show that the evolution of a large amount of oxygen-containing structures plays an important role on the evolution of different types of hybrid carbon atoms during pyrolysis. 3.6. Pore Structure Development of Typical Chars during Activation. To study the pore development under different burnoff values during activation, three typical precursor chars (JX-800, JXO15-800, and JXO48-800) were compared under the same activation conditions. The results of the pore structure analysis of the three porous materials by N2 adsorption are given in Figure 9 and Table 7. The N2 adsorption−desorption isotherm of JX-800-15.5 in panels a and b of Figure 9 can be classified as type I, showing the obvious micropore characteristics. With the increase of burnoffs from 25.9 to 59.1%, the N2 isotherms have exhibited the characteristics of type I at low pressures as well as those of type IV at high pressures and the N2 adsorption capacities also increase constantly at a low pressure, indicating the increase of micropores. The clear hysteresis loop is observed gradually, indicating that the production of some mesopores (2−5 nm) was derived from widening micropores. However, the severe carbon loss at the surface of the particles for JX-800-59.1 can be seen in Figure 10a, although JX-800-59.1 with SBET values of 589.41 m2/ g has formed a hierarchical structure. The changes in the pore structure and surface morphology for JX-800 char under different burnoffs during activation indicate that the pore formation follows the hierarchical development. The diffusion of the activated gas is hindered and follows from the surface to the core for JX-800 char with the limited pores and active sites as well as more ordered crystallites, leading to the occurrence of more reactions at the surface of the particles rather than the interior.

Figure 10. SEM images of JX, JXO15, and JXO48 chars under final burnoff values.

The N2 adsorption isotherms of JXO15-800 and JXO48-800 chars with different burnoff values all have displayed the typical hierarchical characteristics of type I at low pressures and type IV at high pressures in panels c and e of Figure 9. The adsorption capacity of JXO48-800 char increases rapidly at low pressures but has almost no change at high pressures with the increase of burnoffs from 17.1 to 30.5%, which contributes to the development of micropores during the activation stage. A high value of SBET (818.98 m2/g) at a low value of burnoff (30.5%) for JXO48-800 char is observed in Table 7. However, the adsorption capacity of JXO15-800 char exhibits a relatively slow growth trend at low pressures and an obvious growth trend at high pressures with the increase of burnoffs from 26.8 to 42.6%, indicating the obvious development of mesopores during the activation stage. JXO15-800-42.6 with a relatively low value in SBET (705.69 m2/g) is also observed in Table 7. In particular, no severe carbon losses that occur at the particle surface for JXO48800-30.5 and JXO15-800-42.6 are shown in panels b and c of Figure 10. The surface morphology of JXO15-800-42.6 and JXO48-800-30.5 indicates that the developed porous structure of oxidized chars acts as channels to help with the diffusion of activated gas, thereby hindering the severe carbon losses at the particle surface. However, the differences in the microstructure of oxidized chars can affect the pore development during activation. JXO48-800 char produced by a longer oxidization time has formed a large amount of disordered microcrystalline structure and active sites compared to that of JXO15-800 char, which are favorable for rapid production of micropores; however, a rapid development of mesopores in JXO15-800 char mainly results from the widening of the pores.

4. CONCLUSION Physical activation of Jixi bituminous coal upon air preoxidation followed by different oxidation times has demonstrated its potential interest for the preparation of AC with high SBET at low burnoff values. This method provides good control of the structure of Jixi bituminous char to obtain the ideal AC. The air preoxidation stage exhibits an obvious increase of the type and quantity of oxygen functional groups for oxidized coals with the increase of the oxidization time and the depolymerization of the aromatic structure caused by the removal of aliphatic chains; H


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Energy & Fuels

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however, a slight change in the pore structure and surface morphology for oxidized coals is found. In the pyrolysis stage, oxidized chars are transferred into a disordered structure with the increase of the oxidation time compared to ordered conversion for unoxidized char (JX-800); moreover, more active sites and porosity structure (corresponding mainly to meso- and macropores) of oxidized chars are created by the evolution of oxygencontaining structures with different stabilities. The longer oxidation time (96 h) may have a limited role on the further improvement of the oxidized char structure. In the activation stage, the low SBET value (589.41 m2/g) of JX-800-59.1 corresponds to its more ordered structure, which hinders the diffusion of the activated gas, leading to the appearance of serious carbon losses at the surfaces of the particles. Alternatively, the developed porous structures of JXO15-800 and JXO48-800 chars act as channels to promote the favorable diffusion of activated gas, leading to no severe carbon losses at the particle surface. The SBET/burnoff ratios of JXO48-800 char (818.98 m2/g versus 30.5%) obtained are higher than those of JXO15-800 char (705.69 m2/g versus 42.6%) because of more disordered structures and active sites of JXO48-800 char that promote the rapid increase of micropores during activation.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-451-86412618. E-mail: [email protected]. ORCID

Dongdong Liu: 0000-0003-4789-7663 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly thank the financial support from the research project supported by the National Natural Science Foundation of China (51276052).

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Improvement of Activated Carbon from Jixi Bituminous Coal by Air

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