Insight into controllability and predictability of pore structures in pitch-based activated carbons

Accepted Manuscript Insight into controllability and predictability of pore structures in pitch-based activated carbons Taotao Guan, Jianghong Zhao, Guoli Zhang, Dongdong Zhang, Baixin Han, Nan Tang, Jianlong Wang, Kaixi Li PII:

S1387-1811(18)30295-6

DOI:

10.1016/j.micromeso.2018.05.036

Reference:

MICMAT 8938

To appear in:

Microporous and Mesoporous Materials

Received Date: 24 March 2018 Revised Date:

15 May 2018

Accepted Date: 25 May 2018

Please cite this article as: T. Guan, J. Zhao, G. Zhang, D. Zhang, B. Han, N. Tang, J. Wang, K. Li, Insight into controllability and predictability of pore structures in pitch-based activated carbons, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.05.036. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Insight into controllability and predictability of pore structures in pitch-based activated carbons Taotao Guan,

1, 2

Jianghong Zhao,

*, 3

Guoli Zhang,

1, 2

Dongdong Zhang,

1, 2

Baixin Han,

1

Nan

1

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Tang, 1 Jianlong Wang, 1 and Kaixi Li *, 1 Institute of Coal Chemistry, Chinese Academy of Sciences, 27 Taoyuan South Road, Taiyuan,

Shanxi 030001, P. R. China

University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing, 100049, P. R. China

3

Engineering Research Center of Ministry of Education for Fine Chemicals, Shanxi University, 92

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2

Wucheng Road, Taiyuan 030006, Shanxi, PR China *

Corresponding authors

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E-mail addresses: [email protected] (Jianghong Zhao). [email protected] (Kaixi Li).

Abstract: Tailoring porosity of pitch-based activated carbons (PACs) on a large scale is of high importance and usually achieved by introducing additives or templates and adjusting activation

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conditions. Since pitch is a kind of extraordinarily complex mixtures, it is highly necessary to

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investigate the effect of pitch composition on pore structure of PACs. Herein, a coal-tar pitch was subdivided by solvent fractionation into toluene-soluble (TS), pyridine-soluble (PS), quinoline-soluble (QS), and quinoline-insoluble (QI) fractions followed by activation using KOH or steam, respectively, to obtain PACs. It is found that the formation of pores in PACs is significantly dependent on pitch composition and activation methods. The light fractions (TS and PS) are more favorable for pore development in the KOH-activated carbons, whereas pores in the H2O-activated carbons are easily achieved by using the heavy fractions (QS and QI). The possible

ACCEPTED MANUSCRIPT pore-formation mechanisms were also proposed, which indicates that the big differences intrinsically originate from the chemical properties (such as molecular size, structures, reactivity with activation agents and rheological behaviors of the molecular aggregations) of pitch

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compositions. When commercial pitches are employed for the precursors, a synergistic effect on pore development occurs both in KOH- and H2O-activated PACs seriously depending on their mass percentages of light or heavy fractions. It suggests that the mass percentage can be an index

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to foresee the porous properties of PACs. These findings open up a significantly important and

adjusting pitch compositions.

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more practical approach applicable to tailor the pores in PACs on an industrial scale just through

Keywords: Pitch composition; activated carbons; Pore structures; Synergistic effect; Controllability; Predictability.

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1. Introduction

Activated carbons (ACs) have been extensively investigated in many practical applications due to their remarkable properties, such as high surface area, large pore volume, good thermal

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stability, chemical inertness, excellent mechanical stability and easy handling manufacture [1-3].

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Various carbonaceous precursors such as coal [4], pitch [5], polymer [6], biomass [7], metal carbides[8], and so on, can be used for the production of ACs via physical or chemical activation processes[9]. Pitch, a by-product from coal cracking or crude oil distillation industry, is regarded as one of the most important precursors for carbon preparation and has been widely used to produce ACs by virtue of its inexpensive price, low ash, high carbon yields and easily graphitizable feature; thus, pitch-based ACs (PACs) offer numerous advantages over others raw carbonaceous material [5, 10-12].

ACCEPTED MANUSCRIPT The control of pore textures including pore size distribution, pore volume and specific surface area in ACs are highly important for a given application [10, 13]. Considerable research efforts have been devoted to control porosity of PACs through introducing additives or templates

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[14, 15] and/or adjusting of treatment conditions [9, 11, 16]. However, most of them regarded pitch as a monolithic raw material and characterized it by some rough empirical parameters (such as bulk element analysis, softening point, and the solubility in quinoline and toluene) [4, 9, 17-19].

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As-prepared PACs are primarily microporous, which is well-suited to many applications involving

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small molecules, such as molecular sieving, adsorption and catalysis [2, 10, 13, 20, 21]. As a matter of fact, mesopores or even macropores would be more preferable for the adsorption of large molecules (such as vitamins, dyes and polymers) or the diffusion of electrolyte ions [1, 22]. In addition, a predicament was —and still is —that the pitches from different sources often provide

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distinctly different pore textures in spite of the same activation process [23]. This causes the big difficulty in maintaining product stability in industrial manufacture at the moment. Moreover, it is also difficult and time consuming to select a proper pitch precursor for preparing the PACs with

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desirable pore textures. The main reason is that, as a fundamental factor, the effect of pitch

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composition on the pore texture of PACs has been investigated rarely. Since pitch is a kind of extraordinarily complex mixtures undoubtedly affecting the formation

of carbons and thereof pore development, it could be very useful if porous properties can be controlled or predicted facilely through formulating pitch compositions. Therefore, it is highly imperative to delve into the formation mechanism of pores in PACs from both the fundamental research and the practical application points of view. Pitch consists of more than 1,000 polycyclic aromatic hydrocarbons (PAH) compounds [24,

ACCEPTED MANUSCRIPT 25]; it is much difficult and also unnecessary to investigate the role of each exact molecule in the formation of pores in PACs. A feasible and effective way is to subdivide a pitch into various fractions by the solvent fractionation method. Each of subdivided fractions is homogeneous in

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terms of molecular composition and different from the others [26, 27]. The typical cutting method can separate pitches into soluble/insoluble fractions using solvents, including hexane, tetrahydrofuran, toluene, pyridine and quinolone [25, 27-30]. SHISHIDO et al [31] reported that

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large disc-like molecules tend to stack due to attractive intermolecular interactions during

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heat-treatment, while small or non-disc-like molecules do not form oriented structure. Kim et al [28] studied the electrochemical and structural properties of lithium battery anode materials which were obtained by carbonization of several fractions separated by hexane, toluene and N-methyl pyrrolidone derived from petroleum pitch. They found that each fraction with different molecular

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weight affects the coking polymerization during carbonization, thus results in different electrochemical and microstructural properties. Furthermore, Daguerre et al [30] believed that toluene-soluble fraction with small molecular size inhibits the microporous development. Recently,

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Tekinalp et al [10] prepared pitch-based activated carbon fibers from seven pitch fractions, and

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concluded that the dimer molecules with an average molecular weight of 480 Da were preferred for maximizing SSA and pore volumes of PACFs. As such, although researchers have attempted to investigate the relationship between pitch composition and pores of PACs, the available information is still poor.

In the present work, a coal-tar pitch was firstly subdivided by three typical solvents into four fractions i.e. toluene-soluble (TS), pyridine-soluble (PS), quinoline-soluble (QS), and quinoline-insoluble (QI). These solvents differ greatly in molecular composition, polarity and

ACCEPTED MANUSCRIPT solubility, which can separate the parent pitch into four fractions with obviously different molecular weight and chemical structure. The differences of these fractions can represent the typical composition of pitch and be used effectively to further study the effect of main pitch

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composition on the pore texture of PACs. The molecular structure and pyrolysis behaviors of each pitch fractions and microstructures of the corresponding carbon products were studied using FTIR, 13

C-NMR, TG, XRD, Raman and XPS measurements. Through physical (H2O) vs. chemical

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(KOH) activation, the porous structure of as-obtained PACs was measured using N2 adsorption

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isotherm. It shows that the pitch composition significantly affects the formation of pores in PACs activated by H2O- and KOH-activated carbons. It also provides a multi-linear regression (MLR) analysis to foresee the porous features of PACs.

2.1. Chemicals

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2. Experimental section

A mother coal tar pitch (softening point, SP= 274oC) was supplied by Jining Carbon Group

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Co., Ltd. It was fractionated to obtain a series of fractions by ultrasonic-assisted solvent extraction

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technology with toluene, pyridine and quinolone, respectively. A mixture of 20 g mother pitch (sieved to 0.4 mm or less) and 400 mL toluene was heated at 50°C for 4 h under ultrasonic processing. After cooling, addition of toluene and separation of the precipitate by centrifugation were repeated five times. The toluene soluble fraction (TS) was collected by removing the solvent via vacuum distillation. The precipitate was further fractionated with pyridine to obtain pyridine soluble pitch (PS). Similarly, the pyridine insoluble part was fractionated with quinoline to obtain the quinoline soluble fraction (QS) and insoluble fraction (QI). Other four commercial pitches

ACCEPTED MANUSCRIPT were provided by Taiyuan coking plant, Liaoning Ming qiang Co., Ltd and Sichuan Lang zhong Co., Ltd, respectively. 2.2. Preparation of PACs using KOH activation

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As a typical process, H2O, KOH and pitch precursor were firstly mixed up. Then, the mixture was dried at 100oC for 8 h followed by heating to 750oC in the ramp of 3oC min-1 and maintained for 1 h under nitrogen atmosphere. Finally, the product was washed with deionized water to

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remove the alkaline compounds and dried over night at 120oC. The as-obtained PACs were named

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as TS-KOH, PS-KOH, QS-KOH and QI-KOH. An optimized alkali-carbon mass ratio of 4/1 was employed (to be described in electronic supplementary information, ESI, Fig. S1a). 2.3. Preparation of PACs using steam activation.

Firstly, the pitch precursor was crashed and milled. Then, a two-stage process was carried out

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to obtain PACs. It includes the carbonization under a nitrogen atmosphere by heating to 900°C with 3°C min−1 and subsequent steam activation at 900°C for 1 h. The as-obtained PACs were

S1b).

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named as TS-H2O, PS-H2O, QS-H2O and QI-H2O. The activation conditions were optimized (Fig.

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2.4. Characterizations

The softening point was measured according to the method mentioned in the Chinese

standard (GB/T4507-2014). The molecular weight of polymeric naphthalene was analyzed on the ultraflex MALDI-TOF mass spectroscopy (Bruker, Germany) using positive voltage polarity and HCCA as assistant matrix. Fourier transform-infrared (FTIR) spectra of samples were obtained on a Thermo Nicolet-360 spectrometer (USA). The molecular structures of samples were investigated by solid state 13C-NMR spectroscopy on a Bruker AVANCE III NMR spectrometer equipped with

ACCEPTED MANUSCRIPT cross polarization and magic angle spinning. The element content of carbon, hydrogen, nitrogen and sulfur in the samples was analyzed by a Vario Macro EL analyzer (Germany). The oxygen content was calculated by difference. Thermal behavior of the precursors during carbonization was

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analyzed using a HCT-1 instrument (China) under N2 gas flowing and a ramping rate of 10oC min-1. The crystalline structure of the PAC samples was determined via X-ray diffraction (XRD) analysis using a D8 ADVANCE A25 X-ray powder diffractometer (Germany) with CuKα

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radiation. The d-spacing was estimated using the Bragg equation [32]. The crystallite size along

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the c-axis Lc and the lateral size La were calculated by using the Scherrer equation [33, 34]. The IG/ID ratio were calculated by Raman spectra measured by 769G05 laser Raman spectrometer (UK) [35]. The surface area and porosity of PACs were estimated from the isotherms of nitrogen adsorption/desorption at 77 K by ASAP2020 according the Brunauer-Emmett-Teller (BET)

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equation, t-plot method and Barrett-Joyner-Halenda (BJH) method. The pore size distribution of the samples was calculated based on density function theory (DFT) method.

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3. Results and discussion

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3.1. Characteristics of the pitch fractions FTIR spectroscopy is an effective tool to semiquantitatively determine the chemical structure

of pitch. In the FTIR spectra of TS and PS pitch fractions (Fig. 1a), peaks positioned at 1460, 2853, and 2923 cm-1 and peaks at 1380 and 2960 cm-1 correspond to the aliphatic -CH2- and -CH3 groups, respectively [36], indicating that the TS and PS fractions contain many alkyl side chains. The sharp peaks at 880, 824 and 750 cm-1 are ascribed to the out of plane deformation vibrations of isolated aromatic C–H (Ar-H) and Ar-H with 2- and 3-adjacent hydrogens, respectively [27, 37,

ACCEPTED MANUSCRIPT 38], further corroborating the presence of substituents in TS and PS pitch fractions [39, 40]. In addition, a broad band at about 3450 cm-1 and a sharp peak at about 1050 cm-1 in the FTIR spectra of TS and PS fractions suggest the presence of O-H and C-O bonds in the side chains. In contrast,

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the QS and QI fractions have fewer alkyl side chains and Ar-H (most are isolated aromatic C–H) than TS and PS, which related to their larger molecular weight and higher aromatic condensation degree.

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The status of side chains in TS，PS, QS and QI fractions can be analyzed quantitatively by the

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aromaticity index (IAr), which was determined according to the following formula [28, 41, 42]: IAr=A3150-2990/[A3150-2990+A2990-2800], where A3150-2990 and A2990-2800 are the peak area related to aromatic hydrogen and aliphatic hydrogen. The aromaticity index of TS, PS, QS and QI are 0.39, 0.41, 0.44 and 0.46 (Table 1) respectively, implying that the aliphatic groups decrease in the order

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of TS > PS > QS > QI. Furthermore, the length of alkyl side chains can also be evaluated by the ratio of I1460 and I1380, where I1460 and I1380 represent the peak intensities at 1460 and 1380 cm-1 [43]. The results (Table 1) showed that the ratios of TS, PS, QS and QI are 1.15, 1.23, 0.85 and

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0.81, respectively, suggesting that the alkyl side chains of light fractions (TS and PS) are longer

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than that of heavy fractions (QS and QI). The chemical structure of TS, PS, QS and QI fractions was further investigated by CP/MAS

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C-NMR spectroscopy (Fig. 1b). Obvious peaks at 16, 23 and 33 ppm appear in the spectra of

light fractions (TS and PS), revealing the presence of many methylene carbons [44]. In the profiles of heavy fractions (QS and QI), a prominent resonance peak located at 20 ppm and the disappearance/fade of peaks located at 16, 23 and 33 ppm suggest that most of the aliphatic groups in QS and QI fractions are methyl groups connected to the aromatic nucleus [44]. The peak at 41

ACCEPTED MANUSCRIPT ppm, which is assigned to the methylene carbon linking between aromatic nucleuses [45, 46], is becoming evident in profiles of QS and QI fractions, indicating a serious crosslinking degree. In addition, the predominant resonance peaks over 108-129 ppm are usually attributed to the

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pericondensed carbons, protonated carbons and/or non-substituted aromatic carbons. The peaks over 129-148 ppm are ascribed to the catacondensed carbons, outer quaternary carbons and/or substituted aromatic carbons [44, 47, 48]. It is obvious that the side chains of heavy fractions are

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consistent with the FTIR spectra mentioned above.

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shorter than that of light fractions, while the latter possess more side chains. The conclusions are

Generally, soluble fraction consists of lighter molecules while the insolubilized material may be reasonably believed to have higher molecular weight [37, 49]. The increasing of average molecular weight of samples from 327 Da for TS to 483 Da for PS can be clearly seen from the

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MALDI-TOF mass spectrometry, as shown in Fig. S2. Elemental analysis (Table 1) shows that the carbon content increases in the order of TS < PS < QS < QI, whereas the hydrogen content decreases oppositely. The C/H ratio and the softening point both increase in the order of TS < PS <

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QS < QI, confirming the big difference in molecular size and chemical structure of the four pitch

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fractions. According to the results of FTIR, 13C-NMR and elemental analysis, schematic molecular structures of four pitch fractions were presented in Fig. S3. The differences in molecular size and chemical structure could result in distinct aromaticity, rheological behavior and thermal reactivity of pitch [28, 50].

3.2. Carbonization of the pitch fractions. Pyrolysis behaviors of TS, PS, QS and QI pitch fractions were investigated by TGA. As shown in Fig. 2, TG curves of the light fractions (TS and PS) can be divided into three stages,

ACCEPTED MANUSCRIPT while the TG curves of heavy fractions (QS and QI) only have two stages. For the TS and PS fractions, the TG curves begin to lose weight at a much lower temperature of about 60oC, which should be caused by the evaporation of some much lighter compounds and/or the decomposition

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of some unsteady chemicals. With the increasing of temperature approaching to 270oC, a severe weight loss appears and continues up to about 550oC. This can be ascribed to the violent plolycondensation, aromatization and especially the elimination side reactions occurring with the

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evolution of CO2, H2O, CH4 and so on. Since the light TS and PS fractions have smaller aromatic

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nucleus and are rich in aliphatic side chains (Fig. S3), they should be more active and the elimination side reactions would become superior in this stage, resulting in the removal of aliphatic substituents and even skeleton carbons in aromatic nucleus and finally a severe weight loss (much low carbon yield). There is a similar stage in the TG curves of QS and QI fractions, in

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which, however, weight loss is much moderate compared with that of the TS and PS fractions. In view of the chemical structures of QS and QI fractions (Fig. S3), it can be deduced that plolycondensation and aromatization reactions dominate in this stage. DTG peak temperature

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increases in the order of TS < PS < QS < QI, confirming the big differences in reactivity of the

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four pitch fractions. When the temperature is higher than 600oC, all of the TG curves become nearly flat, indicating dominant reactions of rearrangement of carbon skeleton of coking product in carbonization [51-53].

The molecular size, intermolecular forces and planarity could affect arrangement regularity

as well as crystalline structure of the carbonized pitch. Fig. 3a shows the XRD patterns of carbonized TS, PS, QS and QI fractions treated at 900oC. It is clear that all the carbonized samples were not fully graphitized due to the broadened 002 diffraction peak (2θ = 26°) [36]. Structural

ACCEPTED MANUSCRIPT parameters such as interlaminar space (d002), crystal stacking height (Lc) and lateral size (La) were calculated using Bragg equation and Scherrer equation to further compare the microstructure of carbonized products (Table 2). It shows that the microcrystallites in TS- and PS-carbonized

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samples are small-sized with large interlamminar spaces, indicating much turbulent carbon structures. This is because molecular π-π stacking was sterically hindered as a result of the abundant and long aliphatic side chains in the light TS and PS fractions [54] in spite of the low

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softening point beneficial to rheological behaviors during the heat-treatment process. In addition,

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these side chains were removed heavily during the carbonization process of TS and PS in the form of gas at high temperatures, which also lead to an increase in d002 and a decrease in Lc and La [38, 55]. The QS-carbonized sample possesses the smallest interlaminar space and the largest microcrystalline size, indicating that the QS fractions contain large and relatively homogeneous

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polyaromatic molecules (strong π−π interaction) with proper softening point (favorable mobility) which can easily generate large-sized planar macromolecular structures [31, 56, 57]. However, the QI-carbonized sample has moderate interlaminar space and microcrystalline size. This could be

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the result of the high crosslinking degree and the limited molecular mobility (much high softening

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point) of QI inhibit carbon arrangement and lead to a poor crystallization [49]. Raman spectra were employed to investigate the orderness and defect in microcrystallites of

carbonized samples. As shown in Fig. 3b, there are D band at 1345 cm-1 and G band at 1580 cm-1, which are related to the defective carbon structures (C-C sp3 configuration) and the ordered carbon structures (C-C sp2 configuration), respectively [27]. The integrated intensity ratio of the D band and G band (ID/IG) has been used to estimate the degree of perfection of graphene planes [58]. It shows in Table 2 that QS-carbonized sample still possesses the smallest ID/IG value, indicating its

ACCEPTED MANUSCRIPT sp2 carbons are less defective than the other three samples. Chemical composition and type of carbon atoms were determined by XPS [59]. A predominant C1s peak appears at binding energies of ca. 284.5 eV appeared at survey XPS spectra

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(Fig. S4). Further, the deconvoluted C1s spectra (Fig. 4) were fitted into five peaks at 284.6, 285.5, 286.9, 288.6 and 290.1 eV, respectively, which correspond to C-C (sp2 configurations), C-H, C-O, C=O and COOH/COOR [53]. The area ratio of sp2 C-C configurations/other carbon bonds which

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depends on the degree of disorder of carbon materials [59], was calculated from Fig. 4 and shown

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in Table 2. It is clear that the ratios for QS- and QI-carbonized samples are the larger the other two, also indicating well graphitization of the heavy pitch fractions. 3.3. Morphology of the PACs

SEM images show the morphology of PACs (Fig. 5). All of PACs were granular products

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with rough surface. It is noteworthy that the morphology of TS-KOH and PS-KOH PACs is totally different with that of TS-H2O and PS-H2O PACs, while there is no much change in the morphology of QS- and QI-based PACs. The TS-KOH and PS-KOH PACs have abundant round

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holes with diameters of 0.2-2 µm in the carbons, while the TS-H2O and PS-H2O PACs exhibit bulk

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morphology without obvious pores. A possible reason is that during the heat-treatment process, KOH facilitate the volatilization and decomposition of light molecules and the released volatiles and gases can be serving as bubble agents in the favorably mobile TS and PS pitch fractions to create foam cells [60, 61]. Both QS and QI, especially the QS, activated by KOH or steam exhibit stacked layer structures. This is owing to the strong π−π interactions between large polyaromatic molecules through self-assembly [23]. 3.4. Effect of pitch composition on porous structure of PACs

ACCEPTED MANUSCRIPT The above results reveal that the molecular size and structure of pitch precursor are important in determining the microstructure and morphology of carbon materials. It is urgent to investigate their effects on the formation of pores in PACs. The porous texture of PACs was studied by

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nitrogen adsorption isotherms and pore size distribution (PSD) curves. As is shown in Fig. 6a, TS-KOH and PS-KOH exhibit combined characteristics of type I/IV isotherms with a sharply increased adsorption capacity, a wide and inclined knee and an elongated hysteresis loop,

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indicating the presence of considerable micropores and abundant small-sized mesopores [27].

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However, the isotherms of QS-KOH and QI-KOH are typical type I, which include a large nitrogen adsorption below a relative pressure of 0.1 and a plateau at higher relative pressures, indicating that the sample are highly microporous [62]. PSD curves (Fig. 6b) visually exhibit that there are abundant mesopores in size of 2-4 nm formed in the TS-KOH and PS-KOH compared

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with the QS-KOH and QI-KOH.

For the H2O-activated PACs, the isotherm of TS-H2O or PS-H2O (Fig. 6c) shows a very low adsorption capacity, indicating its very poor pore characteristics. Interestingly, the QS-H2O and

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QI-H2O have the combined type I/IV isotherms without an apparent knee at the moderate relative

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pressures, revealing the existence of rich micropores and a few amount of small-sized mesopores. Especially for the QS-H2O, a wide hysteresis loop appears at high relative pressures, indicating large-sized pores formed in the samples. These were confirmed by their PSD curves (Fig. 6d), in which TS-H2O and PS-H2O contains very few pores, whereas QS-H2O own a hierarchical porous texture of micropores as well as mesopores in size of 2~50 nm. Table 3 shows details of the pore parameters of KOH- and steam-activated TS, PS, QS and QI samples. It is clear that KOH activation is more favorable to the formation of pores in both light and heavy pitch fraction-based

ACCEPTED MANUSCRIPT activated carbons than that of steam activation. Furthermore, the light fractions present opposite pore-forming behaviors in KOH and steam activation, while abundant pores can be created in both KOH- and H2O-activated heavy QS- and QI-based PACs. These results suggest clearly that pitch

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composition and activation method have a combined effect on the formation of pores in the PACs. The differences in pore-forming features of the TS, PS, QS and QI pitch fractions during the KOH activation process can be explained by following reasons. It has been proved in the above

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parts that light fractions (TS and PS) possess small aromatic nucleus with abundant aliphatic side

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chains. Moreover, the aliphatic structures are supposed to be probable active sites for KOH [63, 64]. In addition, the small aromatic nucleuses are difficult to be stacked up due to the weak π−π interactions and the flexible mobility. Therefore, this is beneficial to the intercalation and etching of K+ in activation [27, 65, 66]. All these factors result in the formation of rich micropores and

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small mesopores in the TS- and PS-based PACs activated by KOH. On the contrary, the heavy fractions (QS and QI) consist of large polyaromatic nucleus with high aromaticity and low contents of aliphatic side chains, which means strong π−π interactions and lower reaction activity

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with KOH. This can be corroborated by the less weight loss of QS and QI fractions in Fig. 2 and

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the higher carbon yields in QS-KOH and QI-KOH PACs (Table 3). Thanks to the intercalation and etching capability of K+, KOH activation also creates a large number of pores in the QS and QI fractions.

Schemes for the production of H2O-activated carbons involves primary carbonization of the

raw material (i.e. the heating phase) followed by controlled gasification at 900oC in a stream of steam (i.e. the isothermal phase) for an effective activation [67]. As such, for the TS-H2O, PS-H2O, QS-H2O and QI-H2O PACs, the formation of porous structures should be related to the

ACCEPTED MANUSCRIPT microstructures of the carbonized samples and the reactivity of these microstructures with steam [67]. TG curves (Fig. 2) exhibit that during the heating phase, severe reactions including polycondensation, aromatization and the elimination reactions occur at the temperatures of

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270-550oC, much higher than the softening points of TS and PS. It means that the TS and PS fractions have been solidified at this stage, thus large quantities of volatiles and gases released can escape quickly from the bulk, resulting in the bulk morphology significantly different from the

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hole-rich morphology counterparts activated by KOH (Fig. 5). Furthermore, the small

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polyaromatic molecules with rich aliphatic side chains mean small microcrystallites, flexible mobility, high activity and fast reaction kinetics, leading to the formation of densely aggregated bulk carbons with few pores (Table S1 and Fig. S5). According to the similar carbon yields (activation) of steam-activated TS, PS, QS and QI fractions shown in Table 3, H2O molecule react

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with the carbons unselectively, which makes it much difficult to create new pores in the bulk carbons if there are no accessible pores in the carbonized samples. This is like peeling onions, the activation process proceeds through the unselective elimination of active carbon atoms at edges

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and/or defects in the bulk carbons by the H2O molecule, explaining why steam activation creates

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only very few pores in the TS-H2O and PS-H2O PACs. In contrast, heavy QS and QI fractions consist of large polyaromatic molecules with fewer short substituents. The large microcrystallites and poor mobility make it easy to form embedded pores inside of the carbonized samples. Once H2O molecules come into these inner pores, they can create new pores at different active sites, leading to the formation of large amounts of pores in the QS-H2O and QI-H2O PACs. In one word, it is the chemical structure of pitch fractions and the reactivity characteristics of KOH and H2O which determines the formation of pores in the PACs. Based on the discussion

ACCEPTED MANUSCRIPT above, we proposed a schematic diagram shown in Fig. 7, which describes the effect of pitch compositions on the formation of pores in PACs and the possible mechanisms thereinto. 3.5. Porosity of PACs from commercial pitches with different composition.

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In practical, pitches are the mixtures of light and heavy fractions in different mass percentages. It is significantly important to investigate the pore characteristics of mixed pitch precursors. Therefore, five commercial pitches named a, b, c, d and e with different mass content

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of TS, PS, Qs and QI fractions were employed to prepare PACs using KOH or steam activation,

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respectively. Fig. 8a presents mass percentage of the pitch fractions in the five commercial pitches. It shows that the content of heavy fractions (QS+QI) increases in the order of a
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total specific surface area (SSA) and total volume (Vtotal) of the pitch mixed with light and heavy fractions, which should equal to the sum of each pitch fractions. Therefore, a multi-linear regression (MLR) equation was built describing the dependence of SSA or Vtotal (y) on the mass

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percentage of TS, PS QS and QI (X), which is denoted by Eq. (1): ,

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y= AXTS+BXPS+CXQS+DXQI

Eq. (1)

where XTS+XPS+XQS+XQI= 1. The coefficients (A, B, C and D) are the SSA or Vtotal of each pitch fractions shown in Table 3. According

to

Eq.

(1),

the

calculated

SSA

for

a-KOH

PACs

is

equal

to

“2731.7×67.6%+2981.3×13.1%+2513.2×17.3%+2283.8×2%”. Vtotal can be calculated in the same way. The calculated and experimental SSA and Vtotal were displayed in Fig. 8b and 8c. It is clear that the calculated and experimental SSA and Vtotal show a similar variation trend whether in KOH

ACCEPTED MANUSCRIPT activation or steam activation. However, the experimental SSA and Vtotal of a-KOH and b-KOH with light fractions (TS+PS) higher than 50% are much higher than the corresponding calculated values (Fig. 8b). Meanwhile, the experimental SSA and Vtotal of c-H2O, d-H2O and e-H2O, whose

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heavy fractions (QS+QI) are more than 50%, deviate greatly from the calculated values (Fig. 8c). A reasonable explanation is the synergistic enhancement of different pitch fractions. The minor differences in molecule structructures of coal tar pitches from different origins may also be the

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cause of the deviation.

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Furthermore, Fig. 8d shows the relations of SSA and Vtotal of PACs vs. the mass percentage of light fractions (TS+PS) in pitch precursors. With the increasing of the mass percentage of light fractions in pitch, the SSA and Vtotal of KOH-activated PACs increase linearly while that of H2O-activated PACs decrease. Moreover, the nitrogen adsorption isotherms and PSD curves of the

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five commercial pitches (Fig. S6) show genetic behaviors similar to that of the single light and heavy fractions (Fig. 6). These results indicate that pore structures of the PACs can be modulated by controlling the pitch composition and choosing an appropriate activation method. The mass

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percentage of light and heavy pitch fractions can be utilized as an index to foresee and more

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importantly, to control the pores of PACs. Notably, the index is easily available at manufacturing sites of PACs.

4. Conclusions

In summary, TS, PS, QS and QI fractionations were found to be a useful way to modulate

porous structures of the PACs. These pitch fractions show significant differences in terms of elemental composition, molecular size and chemical structure, which in turn influence their pyrolysis and activation behavior, leading to different porous structures. The pores in PACs are

ACCEPTED MANUSCRIPT also dependent on activation methods. There is a synergistic effect on the formation of pores when the pitch fractions are mixed together as the carbon precursor. The pore structures of the PACs can be modulated by controlling the pitch composition and choosing an appropriate activation method.

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This work opens up a more practical way to feasibly control and foresee the pore development of pitch-based activated carbons, which is promising in industrial applications. Acknowledgements

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The authors gratefully acknowledge these financial supports of the National Natural Science

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Foundation of China (Grant Nos. U1510204, 51672291) and the Natural Science Foundation of Shanxi Province for Excellent Young Scholars, China (Grant No. 201601D021006) Notes

The authors declare no competing financial interest.

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(2001) 209-221.

ACCEPTED MANUSCRIPT Figure and table captions

Fig. 1 FT-IR spectra (a) and CP/MAS 13C-NMR spectra (b) of TS, PS, QS and QI pitch fractions.

Fig. 2 TG and DTG curves of TS, PS, QS and QI pitch fractions.

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Asterisks denote the spinning sidebands.

Fig. 3 X-ray diffraction (XRD) patterns (a) and Raman spectra (b) of the carbonized TS, PS, QS

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and QI pitch fractions.

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Fig. 4 Deconvoluted XPS C1s spectra of carbonized pitch fractions.

Fig. 5 SEM images of PACs: TS-KOH (a), PS-KOH (b), QS-KOH (c), QI-KOH (d), TS-H2O (a′), PS-H2O (b′), QS-H2O (c′) and QI-H2O (d′). All the scale bars are 1µm in length. Fig. 6 The N2 adsorption isotherms (a, c) and pore size distributions (b, d) of PACs derived from

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four fractions.

Fig. 7 The possible pore-formation mechanisms of pitch fractions at KOH and H2O activation. Fig. 8 TS, PS, QS and QI mass percentage of five commercial pitch samples (a); SSA and pore

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volume of KOH-activated PACs and (b) H2O-activated PACs (c) determined experimentally and

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those calculated utilizing the Eq. (1); relations showing SSA and Vtotal of PACs vs. the light fraction (TS and PS) content in pitch precursors (d). Table 1 Chemical and structural properties of TS, PS, QS and QI pitch fractions. Table 2 Structural parameters of the carbonized TS, PS, QS and QI pitch fractions. Table 3 The pore structure parameters of PACs derived from four fractions.

ACCEPTED MANUSCRIPT Table 1 Chemical and structural properties of TS, PS, QS and QI pitch fractions. Elemental analysis (wt%) C/Ha Softening point (oC) IArb I1440/I1380c N

S

Odiff.

TS

87.6 5.7

0.9

0.6

5.2

1.28

65

PS

90.4 5.3

0.8

0.4

3.1

1.42

86

QS

92.2 4.9

1.0

0.1

1.8

1.57

176

QI

93.5 4.8

0.8

0.1

0.8

1.62

> 300

1.15

0.41

1.23

0.44

0.85

0.46

0.81

the difference; a the atomic ratio; b the aromaticity index; c the ratio of I1460 and I1380 where I1460

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0.39

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Sample

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and I1380 represent FTIR absorption peak intensities at 1460 and 1380 cm-1.

ACCEPTED MANUSCRIPT Table 2 Structural parameters of the carbonized TS, PS, QS and QI pitch fractions. C-C sp2 bond/other d002 (nm)a

Sample

Lc (nm)a

La (nm)a

ID/IGb carbon bonds c

1.67

3.44

0.851

carbonized PS

0.381

1.82

3.75

0.832

carbonized QS

0.341

2.25

4.64

0.824

carbonized QI

0.355

2.01

4.14

0.841

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a

1.74

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0.383

1.75

1.98

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carbonized TS

1.87

the interlaminar space (d002), the crystal stacking height (Lc) and the lateral size (La)

calculated from XRD pattern; b the ratio ID/IG calculated from the integrated intensity of D band and G band of Raman spectra;

c

the area ratios of chemical bonding peaks calculated from the

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deconvoluted C1s XPS spectra.

ACCEPTED MANUSCRIPT Table 3 The pore structure parameters of PACs derived from four fractions.

Yields

SBET

Smicro

Vtotal

Vmicro

(%)

(m g )

(m g )

(cm g )

(cm g )

TS-KOH

23.5

2731.7

706.2

1.315

0.270

PS-KOH

28.2

2981.3

720.6

1.521

0.331

QS-KOH

56.5

2513.2

1366.9

1.178

QI-KOH

69.3

2283.8

1497.7

1.106

TS-H2O

39.9

52.4

32.0

PS- H2O

35.8

157.9

QS- H2O

34.4

1288.3

QI- H2O

35.0

1271.4

Vmicro/ average pore

Sample 3

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-1

3

-1

Vtotal

width (nm)

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-1

0.21

1.93

0.22

1.94

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2

0.606

0.51

1.88

0.732

0.66

1.85

0.030

0.014

0.47

2.23

76.0

0.091

0.034

0.37

2.30

691.9

0.671

0.312

0.46

2.08

0.616

0.355

0.58

1.92

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2

781.4

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Highlights: · Each of pitch fractions leads to an unique pore structure in activated carbon · Porosity differences originate from the chemical properties of pitch fractions · Light fractions are favorable for pore development in the KOH-activated carbons · Pores in the H2O-activated carbons are easily achieved by using the heavy fractions · Pitch fractions have synergy when they are mixed together as carbon precursor · Foreseeing pore texture of ACs bases on the mass percentage of pitch compositions