Adjusting the Porosity of Coal-Based Activated Carbons Based on a

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Adjusting the porosity of coal-based activated carbons based on a catalytically physical activation process for gas and liquid adsorption Lijie Wang, Fei Sun, Jihui Gao, Xinxin Pi, Zhibin Qu, and Guangbo Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03211 • Publication Date (Web): 01 Jan 2018 Downloaded from http://pubs.acs.org on January 1, 2018

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Adjusting the porosity of coal-based activated carbons based on a catalytically physical activation process for gas and liquid adsorption Lijie Wang, Fei Sun*, Jihui Gao*, Xinxin Pi, Zhibin Qu and Guangbo Zhao School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang, China Corresponding Author: * Fei Sun and Jihui Gao E-mail: [email protected] (F. Sun) and [email protected] (J. Gao)

ABSTRACT: Porous carbons have been widly explored and utilized in the fields of adsorption. Pore structure design is the key for achieving high-capacity and fast adsorption. Herein , we develop a simple and general method for pore regulation of coal-based activated carbons (ACs), which is based on a catalytically physical activation process by using the inherent minerals in coal precursor. By adjusting the inherent minerals distribution in the coal precursors, activated carbons with various pore configurations including microporous structure and hierarchically porous structure can be obtained. More specifically, ZD-HCF-AC from mineral-free coal precursor shows a microporous structure with a low surface area of 345 m2 g-1, while ZD-AC from mineral-rich coal exhibits a hierarchically porous structure with remarkably improved surface area of 933 m2 g-1. Ca and Mg components in the minerals notably promote the development of mesopore and macropores, and lead to the resulting ACs with

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hierarchical pore structure. The effect of inherent minerals is not only limited to Zhundong coal, but also available to other mineral-rich coals. Evaluated as adsorbents, microporous ZD-HCF-AC exhibits excellent SO2 adsorption capacities up to 73.4 mg·g-1; hierarchically porous ZD-AC has the best performance for Rhodamine B (RhB) adsorption with capacity up to 227.8 mg·g-1. This work not only provides a simple and scalable method for preparing coal-based activated carbons for various adsorption applications, but also offers a new route for adjusting the porosity of activated carbons during physical activation process.

Keywords Coal; Inherent minerals, Micropore, Hierarchical pore, SO2 removal, RhB removal

Graphical Abstract


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1. Introduction Porous carbon materials have been long-time considered as promosing adsorbents for gas or liquid adsorption due to their large surface areas, well-developed pore structure and good chemical stability.1-5 For instance, porous carbons have been extensively explored in the gas adsorption/storage, water purification and electrochemical energy storage to solve environmental and energy issues. However, carbon materials in different applications require various structural features. Previous studies have shown that porous carbons with ultra-/micropore structure dominate as gas storage (e.g., SO2 adsorption, H2S removal, CO2 capture and H2/CH4 storage) materials.6-7 While, porous carbons used for water purification (e.g., dye or pigments removal) require hierarchical pore structure with micro-/mesopore. 8-9 In order to enhance adsorption performance of porous carbon, many researchers concentrate mainly on the construction of developed, interconnected and appropriately distributed pore configurations with high surface area or large pore volume. Nowadays, various techniques, such as physical or chemical activation methods,10-11 soft/hard template methods,3,12 hydrothermal carbonization methods,1 self-assembly methods13 have been reported for the synthesis of porous carbon materials with a remarkable diversity of structure and property. Among a wide range of porous carbons such as biomass-derived activated carbons, ordered mesoporous carbons and MOF-derived porous carbons, coal-based activated carbon/cokes have been regarded as the most economical adsorbents, which have been utilized in gas pollutant SO2, NOx and dye


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removal.2,9 Besides, the preparation of activated carbons is commonly achieved by physical activation, employing H2O, CO2, flue gas or their mixtures as activation agents, in which process gas components can partially etch the carbon-based coal framework to create desirable porosity. Therefore, physical activation process is considered the simplest, the most effective, and the most economical method. Previous studies have suggested that the inherent minerals in raw coals including alkali,14-16 alkaline earth metals (AAEM)17-19 and transition metal20 pose significant impacts on the reaction between active gas and coal matrix by providing catalytic active sites. Such catalysis effect could greatly boost the porosity development of resulting coke.21 Due to the similarity of physical activation reaction and gasification reaction, we believe that the existence of AAEM species could also affect the pore structure of resulting activated carbons. Zhundong coal, a kind of low-rank coal, is recently discovered in the eastern Xinjiang Zhun GeEr district, which has a largely exploitable reserve of 390 Gt.22 Zhundong coal possesses multiple features including lower ash and sulfur content, medium calorific value and higher volatile matter, which make it suitable for the preparation of coal-based activated carbons.6 It’s also worth mentioning that catalytic metal components constitute the majority in the total ash, especially Ca, Na and Mg. Thus, Zhundong coal provides an applicable platform to investigate the effect of inherent AAEM species of the development of pore structure of resulting activated carbons. In this work, we systematically investigate the effects of inherent minerals on


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porosity development of Zhundong coal-based activated carbons. Physicochemical property characterization results suggest that the metal species pose a significant impact on the porosity development. By removing or adding some metal species in the precursor, microporous carbons and hierarchically porous carbons can be successfully prepared. To verify the application potentials of resulting activated carbons with various pore structure, SO2 and RhB removal tests were conducted, which demonstrates the as-obtained microporous carbons are suitable to be SO2 adsorbents and the as-obtained hierarchically porous carbons are applicative for RhB removal.

2. Materials and methods

2.1. Raw materials Zhundong (ZD) coal, a kind of sub-bituminous coal, was employed as a carbon source. The proximate, ultimate and ash composition analyses of ZD coal are listed in Table 1and Table 2. Table 1. Proximate and ultimate analyses of ZD coal.

Proximate analysis (wt.%)

ad

Ultimate analysis (wt.%, daf)

Mad

Aad

FCad

Vdaf

C

H

O*

N

S

11.79

3.68

56.64

32.70

73.52

6.55

18.51

0.91

0.51

Air-dried basis, d dry basis, daf dry and ash-free basis, * by difference.

Table 2. Ash composition analyses of ZD coal.

Ash composition analysis (wt.%) SiO2

Al2O3 Fe2O3

TiO2

CaO

MgO

K2O

Na2O MnO2

SO3

P2O5

13.33

10.71

0.47

37.75

9.98

0.62

9.78

6.52

0.19

6.19


0.16

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2.2. Preparation of demineralized ZD coal samples To investigate the effect of the inherent minerals on pore configuration and SO2 adsorption properties of coal-based activated carbons, ZD coal was demineralized with hydrochloric acid (HCl), hydrofluoric acid (HF) or the combination of HCl and HF to obtain the demineralized ZD coal with various mineral retaining. More specifically, to remove Fe, Al, alkali and alkaline-earth metals (AAEM), ZD coal was mixed with 5M HCl (1g/100 mL) in a beaker with stirring at 50 °C for 12 h. After filtration and washing with deionized water until neutral PH value, the resulting dried sample was named as ZD-HC. Alternatively, ZD coal was treated with 20% HF under the same heat treatment to remove the most Si compositions but remain the AAEM and Fe. The resulting coal sample with HF treatment was denoted as ZD-HF. To remove all the minerals in ZD coal, sample ZD-HC was treated with 20% HF to obtain the ZD-HCF sample.23 From the XRD patterns shown in Figure S1, minerals in the raw coal were nearly all washed off by HCl and HF solution.

2.3. Preparation of mineral-loaded coal samples Sample ZD-HCF, which could be seen as ash-free coal, was chosen to load with extra minerals. A set of CaCl2-loaded coal sample was prepared by liquid impregnation. A known amount of CaCl2 solid was firstly added into a beaker, and dissolved by deionized water under magnetic stirring. A pre-weighed amount of mineral-free coal was then added into the beaker to make coal-water slurry. The coal-water slurry was dried at 80oC with magnetic stirring of 300r·min-1, until the water was completely evaporated. The Ca-loaded coal sample was denoted as ZD-HCF-Ca. ZD-HCF-Mg was prepared by mixing ZD-HCF with the MgCl2 solution as ZD-HCF-Ca. The mass 6 ACS Paragon Plus Environment

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content of Ca or Mg in the mineral-loaded coal samples is controlled at ~5wt.%.24

2.4. Preparation of activated carbons (ACs) The activated carbons (ACs) were obtained using physical activation in a horizontal tube furnace, as shown in Figure S2. For each run, 3g of above-mentioned coal samples were heated at a constant rate of 10oC min-1 to 900oC and kept for 1h with a total flow rate of 200mL·min-1 (including 120mL·min-1 N2 gas and 80mL·min-1 CO2 gas). The obtained samples were subjected to acid wash with 2M HCl aqueous solution at 60oC for 12 h, and then washed with deionized water till the pH value of the washing solution reached ca.7 with almost no residual chlorine (Figure S3), finally dried at 80oC for 12h under a dryer. According to the coal samples, as-obtained coal-based activated carbons were named as ZD-AC, ZD-HC-AC, ZD-HF-AC, ZD-HCF-AC, ZD-HCF-Ca-AC and ZD-HCF-Mg-AC respectively. XRD (Figure S4) and XPS (Figure S5) analysis of activated carbons before and after washing have shown that the finally obtained-ACs have almost no residual metal species.

2.5. Characterization N2 adsorption/desorption isotherms of the resulting ACs were measured at -196oC using ASAP 2420 V2.05 apparatus. The BET surface area was calculated from the isotherm using the Brunauer-Emmett-Teller equation.25 The micropore volume (Vmic), was estimated by the Horvath–Kawazoe (HK) method.26 The mesoporous volumes (Vmes) and pore size distributions of samples have been calculated by applying Barrett– Joyner and Halenda (BJH) method. The total pore volume (Vtotal) was calculated from the N2 amount adsorbed at relative pressure of 0.975. The pore size distribution was calculated using nonlocal density founctional theory (NLDFT) method by the


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adsorption branch. Scanning electron microscopy (SEM, Helios Nanolab600i) with energy dispersive X-ray apparatus and transmission electron microscopy (TEM, JEM-2100) were adopted to record the morphology and inherent mineral distribution of the samples. X-ray diffraction (XRD) patterns were measured on a Rigaku D/Max 2400 diffractometer using CuKa radiation (40 kV, 40 mA, λ=1.5406 Ǻ). X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI 5700 ESCA System with Al Ka X-ray at 14kV and 6mA. Thermogravimetric analysis was conducted to record the weight loss behaviors during physical activation process using the same gas atmospheres as preparation procedure. Typically, 10mg of coal samples were heated at a rate of 10oC·min-1 until 900oC holding 1h, with 40mL·min-1 CO2 and 60mL·min-1 N2 flowing through. Carbon conversion during thermogravimetric analysis was calculated by the following eq 1.27

=

(1)

Where is carbon conversion, W is the initial sample mass, W is the sample mass at time t. Reactivity of activated carbon was evaluated by normalized reaction rate,28 and it was calculated by eq 2.

=

⁄

(2)

Where is the normalized reaction rate (min -1), ⁄ is the reaction rate, and the

is the carbon conversion at time t.

2.6. SO2 adsorption test SO2 removal tests for coal-based activated carbons were carried out using a self-made 8 ACS Paragon Plus Environment

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fixed-bed experimental system, Portable FTIR (GASMET Dx4000) was used to continuously monitor the SO2 concentrations, as shown in Figure S6. In each typical running, 0.5g of coal-based activated carbons was put into the glass column. Subsequently simulated flue gas (2500 ppm SO2, N2 balance, total flow rate 200 mL min-1) was introduced into the reactor at room temperature. SO2 removal efficiency and rate versus time were evaluated by detected concentrations of SO2 at the inlet and outlet in real time through the gas analyzer. SO2 removal capacity of coal-based activated carbons was calculated by the integrating area above the removal curves and reaction time.29

2.7 Rhodamine B adsorption test The liquid phase pollutant removal study was performed by batch sorption experiments using Rhodamine B (RhB) as an adsorbate (Figure S7), the chemical structure and properties of RhB are shown in Figure S830 and Table S18, respectively. The RhB solution concentration of 200mg L-1 was configured and subsequently diluted to required concentrations (2.5-30 mg L-1). The absorbance of the RhB was detected by UV-visible spectroscopy (T6, Beijing Purkinje General Instrument co., Ltd). Calibration curve for RhB was prepared by recording the absorbance values for a range of known concentrations at the wavelength of maximum absorbance, and the value, λmax, was found to be 554nm for RhB.9 The detailed data were listed in Table S2 and Figure S9. It can be seen that in a certain range of concentration change (0-20mg L-1), the relationship between absorbance values and a certain range of concentration (0-20mg 9 ACS Paragon Plus Environment

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L-1) of RhB are linear (Abs=0.0286+0.0838X, X is the concentration of RhB, in the 0-20 range). Therefore, the detected RhB solution was diluted to a range of concentration from 0 to 20 mg L-1. Adsorption experiments were conducted by adding 20mg of coal based porous carbons into 50ml RhB solution with each initial concentration of 200mg L-1 with stirring at 300rpm. After adsorption equilibrium, 0.4 mL of RhB solution were took out and diluted 10 times, then separated by centrifugation at 5000 rpm for 2 min. The absorbance of clarified supernatant solution was analyzed using a UV-visible spectroscopy mentioned above. 3. Results and discussion

3.1 Mineral composition of raw or demineralized coal samples Figure 1 shows the metal distribution profile of coal samples with various pre-treatment processes. For the raw ZD coal, Ca is the most abundant metal element among all the alkali and alkaline earth metals (AAEMs), followed by Na and Mg, which is consistent with the data in Table 2. HCl treatment could remove over 90% of the Ca, Mg, K, and Na31 and endows the resulting ZD-HC sample with negligible metal species. For the HF treatment case, the resulting ZD-HF sample retains high Ca, Mg and Al contents, getting close to those in ZD, due to the fact that the fluoride of AAEM, Fe and Al are insoluble in water. In addition, HF could remove Si element in the coal samples, so the content of Si in the ZD-HF has a significant decrease. The combining treatment of HCl and HF removes almost all the metal species. Thus, various de-mineralized coal 10 ACS Paragon Plus Environment

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samples with different metal species distributions have been successfully obtained using different acid treatment procedures, which provide a suitable precursor platform for investigating the effects of metal species on the porosity development of resulting activated carbons.

Figure 1. Metal species contents in various coal samples.

Ash composition analysis methods are presented in the supporting information 3.2 Surface morphology and chemical structure of ACs The surface morphology and microstructure of the coal-based activated carbons were investigated by both scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), as shown in Figure 2. The SEM images of various coal-based ACs (Figure 2a~d) exhibit analogously compact stacking structure, which is derived from the morphology of the parental raw coal. However, the HRTEM images in Figure 4e~h indicate the difference between various ACs, particularly, the nano-scale pores distribution within the carbon framework. Compared to ZD-HCF-AC 11 ACS Paragon Plus Environment

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(Figure 2g) and ZD-HC-AC (Figure 2h) with only micropore distribution, there are apparent mesopores existing in the sample ZD-AC (Figure 2e) and ZD-HF-AC (Figure 2f). Considering the only difference between various coal precursors, coal sample ZD and ZD-HF retain the metal minerals while ZD-HC and ZD-HCF remove most of the metal species, especially AAEMs (Figure 1). Thus, the SEM and HRTEM results suggest that inherent minerals in the coal samples enhance the development of meso-/macro-pores the resulting activated carbons (ZD-AC and ZD-HF-AC), which will be further confirmed by the pore structure analysis. Also, all the HRTEM images demonstrate the amorphous nature of as-obtained ACs, which accords well with X-ray diffraction (XRD) patterns (Figure S11). Prepared ACs exhibits two broad diffraction peaks at ca. 24o and 42o assigned as the (002) and (100) diffraction planes of amorphous carbon,32-33 and the two peaks for ZD-AC became less visible in comparison to other samples.

Figure 2. SEM (a, b, c and d) and TEM (e, f, g and h) images of coal-based activated carbon: (a and e) ZD-AC; (b and f) ZD-HF-AC; (c and g) ZD-HCF-AC; (d and h) ZD-HC-AC.

X-ray photoelectron spectroscopy (XPS) analysis was conducted to further 12 ACS Paragon Plus Environment

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investigate the surface chemical compositions of as-prepared ACs from various coal samples. Figure 3a shows the XPS survey spectra of ACs, revealing the similar element types including C, and O between raw coal and demineralized coal based ACs, which suggests that the composition and content of the minerals in coals have less effect on surface chemistry of resulting ACs. Further XPS element content analysis reveals the different oxygen percentages between various ACs. More specifically, ZD-HCF-AC had the highest oxygen content of 6.88 at%, followed by ZD-HF-AC and ZD-HC-AC, while ZD-AC has the lowest oxygen content of 3.2 at%, as shown in Figure 3b. This is in agreement with previous studies that acid pre-treatment of coals could create edge-plane-like defect sites and decorate these sites with extra surface oxygen functional groups.34 The high-resolution O1s spectrum of sample ZD-AC is shown in Figure 3c, which can be similarly fitted by five individual component peaks corresponding to C=O(carboxyl) (530.6±0.2eV), C=O(ester, amides) (532.3±0.3eV), C-O-C(ester oxygen) (533.5±0.2eV), COH, COOH, N-O-C (534.3±0.2 eV), and H2Oads, O2ads (363.3±0.4 eV).35 The atomic percentages of various oxygen- containing functional groups in ACs are summarized in Figure 3d, from which the higher amounts of oxygen-containing groups in ZD-HCF-AC, ZD-HC-AC and ZD-HF-AC are mainly attributed to the increase of carboxyl and ether bonds.


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Figure 3. (a) XPS survey spectra of ACs; (b) Oxygen percentages of ACs; (c) high-resolution O1s spectrum of ZD-AC. (d) Atomic percentage distribution of the four major O species of ACs.

3.3 Pore structure of ACs Nitrogen adsorption/desorption isotherms and corresponding pore size distributions were further obtained to analyze the pore structure of various coal sample derived ACs, as shown in Figure 4, which demonstrates significant differences. ZD-HC-AC and ZD-HCF-AC display a typical Type I adsorption isotherm with significant nitrogen uptake at a relative pressure less than 0.1 indicating the presence of a large fraction of micropores.36 The development of micropores was attributed to the reaction between coal-based carbon framework and CO2 gas at high temperature during physical activation process.5 By contrast, ZD-AC and ZD-HF-AC exhibit combined


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characteristics of type I/IV curves, with strong N2 adsorption at below P/P0 = 0.01 and an obvious loop at a relative pressure of 0.5~1.0, indicating the co-existence of the presence of micropores, mesopores and macropores.37-38 These results are consistent with above-mentioned HRTEM analysis. The pore size distributions shown in Figure 4b further confirm the narrow micropore diameters below 2 nm for ZD-HCF-AC and ZD-HC-AC, and the hierarchical pore configurations (50nm for macropores) for ZD-AC and ZD-HF-AC. The detailed pore parameters (BET surface area, pore volume values) of the AC samples are listed in Table 3, which also shows distinct differences between various ACs.

Figure 4. (a) N2 adsorption/desorption isotherms of various ACs. (b) Corresponding pore size distributions of various ACs.

As can be seen from Table 3, the inherent minerals in precursor coals pose a significant impact on the pore structure of resulting ACs. In particular, ZD-AC from mineral-rich ZD coal exhibits a high surface area (933 m2 g-1) and a high total volume (0.70 cm3 g-1) while the ZD-HCF-AC from mineral-free ZD-HCF shows a much lower 15 ACS Paragon Plus Environment

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surface area and pore volume of 345 m2·g-1 and 0.19 cm3·g-1, respectively. Figure 5 further shows the specific surface area and pore volume distributions between various ACs. AC samples possess obvious different surface area and pore volumes, especially between mineral-rich coal based AC and mineral-free coal based AC. For instance, the ratio of micropore surface area (Figure 5a) of ZD-HCF-AC and ZD-HC-AC are 0.90 and 0.88, higher than those of ZD-AC(0.47) and ZD-HF-AC(0.71), while the ratios of meso-/macro- pore volume (Figure 5b) of ZD-AC and ZD-HF-AC are 0.63 and 0.61, respectively, much higher than those of ZD-HC-AC(0.17) and ZD-HCF-AC(0.15), clearly demonstrating that inherent metal species in ZD and ZD-HF coals promote the development of mesopores and/or macropores during physical activation process. Table 3. Pore structure parameters of as-prepared ACs

Sample

2 -1 SBET (m2 g-1) Smic (m g )

Vtotal (cm3 g-1) Vmic (cm3 g-1) Vmec+mac (cm3 g-1)

Dave(nm)

ZD- AC

933

438

0.70

0.23

0.44

2.99

ZD-HCF-AC

345

311

0.19

0.16

0.03

2.19

ZD-HC-AC

363

318

0.20

0.17

0.03

2.21

ZD-HF-AC

426

304

0.40

0.16

0.24

3.74

ZD-HCF-Ca-AC

289

100

0.23

0.05

0.18

3.19

ZD-HCF-Mg-AC

436

331

0.25

0.19

0.05

2.28


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Figure 5. Specific surface area (a) and pore volume (b) distributions of prepared ACs.

In order to further explore the relationship between the pore structure and the mineral types and contents, we carried out correlation analysis by correlating the mineral content and pore volumes, as illustrated in Figure S12. It can be observed that the micropore volume (Vmic) shows no obvious correlation with the mineral contents, while the meso-/macro-pore volume (Vmec+mac) is closely associated with mineral contents in coal samples, especially with the contents of Ca and Mg. Hereto, we clearly demonstrate that the inherent mineral in precursor coals, especially Ca and Mg compositions, play a dominant role in developing high porosity activated carbons, particularly, the development of mesopore and macropores. Figure 6 shows the SEM images of non-washed ZD-HCF-AC (Figure 6a) and non-washed ZD-AC (Figure 6b) samples, as well as the corresponding EDX image (Figure 6c-d).39 For the non-washed ZD-HCF-AC sample, a smooth surface can be observed, indicating the treatment of HCl and HF remove most of the minerals in raw coal. By contrast, there are many white nano-particles distributing in the surface of non-washed ZD-AC, which can be indexed to various metal species, especially the peak of Ca species was obvious. 17 ACS Paragon Plus Environment

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Figure 6. (a) SEM image of non-washed ZD-HCF-AC. (b) SEM image of non-washed ZD-AC. (c-d) SEM and corresponding EDX image of non-washed ZD-AC.

For further demonstration of the effect of Ca and Mg component, we also prepared Ca-loaded coal sample and Mg-loaded coal sample by extra loading a certain amount of Ca or Mg (5wt.% CaCl2 or 5wt.%MgCl2) into the mineral-free coals. The resulting activated carbons are named as ZD-HCF-Ca-AC and ZD-HCF-Mg-AC. As expected, both ZD-HCF-Ca-AC and ZD-HCF-Mg-AC show hierarchically porous structures (Figure 4) with higher ratios of meso-/macro-pores (Table 3 and Figure 5) in comparison with ZD-HCF-AC. To further verify whether inherent minerals in other types of coals play a role in enhancing pore development. We employed another kind of coal, namely Huolinhe lignite (HLH) from Chinese Inner Mongolia Huolinhe region, to prepared activated carbons. The industrial, ultimate analysis and ash analysis are shown in Table S6 and Table S7, which show the mineral rich characteristics. Experiencing different acid washing procedures or Ca/Mg (5wt.% CaCl2 or MgCl2) loading procedures, the HLH coal based activated carbons show similar pore variation trends (Figure S13, and Table S8) as those of ZD coal based activated carbons, directly demonstrating that the effect


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of inherent minerals on pore development is not only limited to Zhundong coal but also available to other mineral-rich coals. Besides, excessive amounts of minerals in the coal could cause serious ablation and damge on the pore structure during activation process. 3.4 Reactivity of variously treated coal samples with CO2 To explore the catalytically physical activation mechanism, thermogravimetric analysis (TGA) was conducted under a simulated physical activation condition and the results are shown in Figure 7a. ZD and ZD-HF display similar mass loss trends with over 90% weight losses in short time (ca.20 minutes) after the reaction temperature reaching 900oC, demonstrating the high reactivity and reaction rate of mineral-rich coals.13,20,40-42 In comparison, mineral-free coal samples (ZD-HCF and ZD-HC) exhibit relatively lower weight losses (ca. 56%-60%) rates, indicating a lower reactivity. It is also worth mentioning that the Ca loaded coal (ZD-HCF-Ca) shows similarly high weight loss rates (ca. 90%) with ZD and ZD-HF, and Mg loaded coal (ZD-HCF-Mg) shows higher weight loss rates (ca. 70%) in comparison with ZD-HCF. Figure 7b further shows the average reaction rate in TGA from 90 to 100 min between various coal precursors. Various coal samples possess obvious different reaction rates, especially between mineral-rich coals and mineral-free coals. Thus, the different TGA behaviors indicate that mineral-rich precursor coals exhibit high reactivity during the physical activation and thus endows the resulting ACs with highly developed hierarchical pores. For instance, the reaction rates of ZD and ZD-HF are 0.185 and 0.042, higher than those of ZD-HCF(0.007) and ZD-HC(0.008). Besides, TGA 19 ACS Paragon Plus Environment

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behaviors of coal samples in pure CO2 atmosphere (Figure S14) have appeared a similar tendency as in 40vot-% CO2 atmosphere, deeply indicating Ca and Mg species could catalyze the etching reaction. Similarly, thermogravimetric behaviors of HLH coals with various treatment processes also highlight the enhancing effects of inherent minerals, particularly the Ca’s effect, on the coal reactivity (Figure S15).

Figure 7. (a)TGA profiles of various coal samples under simulated physical activation condition (10oC min-1 up to 900oC; holding 1h in CO2 and N2 atmosphere with a flow rate of 100 mL min-1; volume ratio of CO2:N2=2:3). (b) The average reaction rate in TGA from 90 to 100 min.

3.5 Pore development mechanism by catalytically physical activation Hereto, we clearly demonstrate the catalytic effects of inherent minerals on the porosity development of coal-based activated carbons during physical activation. As shown in Figure 8, the pore development mechanism by catalytically physical activation is illustrated. For the mineral-rich coal, the minerals in coals, especially Ca and Mg species, could catalyze the etching reaction between carbon matrix and activation agent (e. g. CO2) and hence promote the porosity development, which leads to the resulting activated carbon with a hierarchically porous structure. For the mineral-free coal, the 20 ACS Paragon Plus Environment

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coal just experiences a traditional physical activation process, which gives a micropore-dominant activated carbon with limited surface area.

Figure 8. Pore development mechanism by catalytically physical activation

3.6 SO2 adsorption properties of ACs To evaluate the application potentials of as-obtained Zhundong coal based ACs as gaseous adsorbents, we further carried out the SO2 dynamic adsorption test. The SO2 removal properties of as-obtained ACs are displayed in Figure 9. Figure 9a and b presents SO2 adsorption efficiency and cumulative adsorption capacity versus time under room temperature and 1 bar in the SO2+N2 atmospheric condition. The difference between various ACs samples result in the SO2 adsorption capacities following the order: ZD-HCF-AC > ZD-HC-AC > ZD-HF-AC > ZD-AC. Specifically, the SO2 adsorption capacity of ZD-HCF-AC after 48min reaches 73.4mg·g-1, which is much 21 ACS Paragon Plus Environment

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higher than other prepared ACs and reported activated carbon materials for SO2 adsorption (Table S9),29,43-45 indicating that micropore structures were in favor of increasing the SO2 adsorption capacity.43 For further demonstration, we also investigate the relationships between SO2 removal properties and pore structure of as-obtained ACs, as shown in Figure 9c. As can be seen, the amount of SO2 adsorption increased with the ratios of micropore surface area but decreased with the ratios of meso-/macro- pore volumes.44 Meanwhile, as shown in Figure 9d, SO2 adsorption capacities show no correlation with the oxygen-containing groups of ACs. Since the higher amounts of oxygen-containing groups in ZD-HCF-AC, ZD-HC-AC and ZD-HF-AC are mainly attributed to the increase of carboxyl and ether bonds, we further study the relationship between SO2 adsorption capacities and the amount of carboxyl or ether bonds, as shown in Figure S16. It can be seen from Figure S16a, SO2 adsorption capacities almost linearly increase with the amount of oxygen functional group including COH, COOH, N-O-C, which was consistent with our previous research.46 While, SO2 adsorption capacities show no correlation with the ether bonds of ACs (Figure S16b).


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Figure 9. (a) SO2 dynamic adsorption curves of various ACs; (b) SO2 accumulated adsorption capacities versus time. Inset shows the sulfur capacities of various ACs; (c) The relationships between sulfur capacities and pore structure various ACs. (d) The relationships between sulfur capacities and oxygen percentages of various ACs.

3.7 RhB removal properties of ACs Porous carbons with abundant mesopores or macropores are commonly considered favorable for the adsorption of liquid phase macromolecular pollutants.47 We further explore the application potentials of as-prepared ACs as liquid adsorbents using RhB as an probe macromolecular pollutant. Figure 10 shows the RhB removal properties of various ACs. The absorbances of dilutedly detected RhB solution versus time of ACs are displayed in Figure 10a, and their corresponding adsorption capacities are shown in 23 ACS Paragon Plus Environment

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Figure 10b. As can be seen, ZD-AC with a hierarchical pore structure shows the best adsorption capability with the RhB adsorption capacity of 227.8 mg·g-1. Such high RhB adsorption capacity is also among the highest levels of reported activated carbon materials8,9,48-50 (A comprehensive comparison shown in Table S10). Compared with ZD-AC, ZD-HCF-AC and ZD-HC-AC have little adsorption capacities due to their microporous characteristics with small pore size, hindering RhB’s access.8 Figure 10c shows the relationships between RhB removal properties and pore parameters of as-obtained ACs. RhB adsorption capacities increase with the increase of the ratio of meso-/macropores, clearly demonstrating the enhancing effects of hierarchical pores on RhB adsorption. Meanwhile, Figure 10d shows that RhB adsorption capacities had no significant relation with the amounts of oxygen-containing groups.


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Figure 10. (a) The absorbance of dilutedly detected RhB solution versus time of various ACs; (b) RhB accumulated adsorption capacities versus time of various ACs; (c) The relationships between RhB adsorption capacities and pore structure of various ACs. (d) The relationships between RhB adsorption capacities and oxygen percentages of various ACs.

4. Conclusion In summary, we have systematically investigated the effects of inherent minerals in Zhundong coal on the porosity development of resulting activated carbons. By adjusting the inherent minerals distribution in the coal precursors, activated carbons with various pore configurations including microporous structure and hierarchically porous structure can be obtained. ZD-HCF-AC from mineral-free coal precursor shows a microporous structure with a low surface area of 345 m2 g-1, while ZD-AC from un-treated coal precursor exhibits a hierarchically porous structure with much-improved surface area of 933 m2 g-1. The meso-/macro-pore volume of ACs is closely associated with mineral contents in coal samples, especially with the contents of Ca and Mg. Thermogravimetric analysis indicates that the role of inherent minerals in promoting macro-mesoporous development could be attributed to the enhanced gasification reactivity during the physical activation process. Evaluated as adsorbents for SO2 removal, microporous ZD-HCF-AC exhibits excellent capacities up to 73.4 mg g-1; evaluated as RhB adsorbents, hierarchically porous ZD-AC has the best performance with RhB adsorption capacity up to 227.8mg g-1 after 91min. This work provides a simple and scalable method for preparing coal-based activated carbons for various adsorption applications 25 ACS Paragon Plus Environment

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including gas and liquid adsorption. Besides, such catalytically physical activation effects of inherent minerals on the porosity development is not only limited to Zhundong coal but also available to other mineral-rich coals. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (51376054 and 51276052). References (1) Hu, B.; Wang, K.; Wu, L. H. Engineering carbon materials from the hydrothermal carbonization process of biomass. Adv. Mater. 2010, 22(7), 813-828. (2) Alabadi, A.; Razzaque, S.; Yang, Y. W. Highly porous activated carbon materials from carbonized biomass with high CO2 capturing capacity. Chem. Eng. J. 2015, 281,606-612. (3) Nishihara, H.; Kyotani, T. Templated nanocarbons for energy storage. Adv. Mater. 2012, 24(33), 4473-4498. (4) Yan, Z.; Liu, L.; Zhang, Y.; Liang, J.; Wang, J.; Zhang, Z.; Wang, X. Activated semi-coke in SO2 removal from flue gas: selection of activation methodology and desulfurization mechanism study. Energy Fuels 2013, 27 (6), 3080-3089. (5) Zhu, Y.W.; Gao, J. H.; Li, Y. Preparation of activated carbons for SO2, adsorption by CO2, and steam activation. J. Taiwan Inst. Chem. 2012, 43 (1), 112-119. (6) Sun, F.; Gao, J. H.; Yang, Y. Q. One-step ammonia activation of Zhundong coal generating nitrogen-doped microporous carbon for gas adsorption and energy storage. 26 ACS Paragon Plus Environment

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Adjusting the Porosity of Coal-Based Activated Carbons Based on a

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