Influence of chemical modification of anthracite on the porosity of the resulting activated carbons

Carbon 40 (2002) 1287–1294

Influence of chemical modification of anthracite on the porosity of the resulting activated carbons b, * ´ S.B. Lyubchik a , R. Benoit b , F. Beguin a

Institute of Physical Organic and Coal Chemistry, National Academy of Sciences, R. Luxemburg Strasse 70, 340114 Donetsk, Ukraine b ` Divisee ´ , CNRS-University, 1 B, Rue de la Ferollerie ´ ´ Centre de Recherche sur la Matiere , 45071 Orleans Cedex 02, France Received 9 March 2001; accepted 20 October 2001

Abstract Activated carbons in a wide range of porosity (from essentially microporous to essentially mesoporous) have been prepared from anthracite by the combination of a chemical treatment with HClO 4 or Mg(ClO 4 ) 2 and a physical activation with CO 2 at 8508C. The main effects of the chemical treatment are a modification of anthracite microstructure by insertion and oxidation. The extent of these two reactions was found to be related to the nature of the chemical agents, the treatment time and temperature, and the textural properties of anthracite. As a consequence, the pretreatment of anthracite causes a noticeable reduction of the activation time and a change of the final pore size distribution. The influence of chemical treatment parameters has been analysed by means of XPS, FTIR, TGA, mass spectrometry, elemental analysis and gas adsorption techniques. The optimal conditions for producing activated carbons from chemically modified anthracites were identified. Step by step increasing of the temperature of anthracite treatment by HClO 4 up to 1608C seems to be the best way to obtain a precursor of highly activated carbon with well-balanced micro and meso porosity.  2002 Published by Elsevier Science Ltd. Keywords: A. Activated carbon, Coal; B. Chemical treatment; D. Reactivity, Porosity

1. Introduction The technologies related to pollution abatement require very specific adsorbents applicable either in liquid or in gas phase processes. Among them, activated carbons are very interesting for the various possibilities of adjusting their properties [1,2]. The adsorption capacity of activated carbons is determined by their microtexture (mainly by the pore size distribution) and is also strongly influenced by the surface functionality [2,3]. The activation process together with the nature of the precursor strongly determine the characteristics of the resulting activated carbons. Among a wide range of precursors for activated carbons, anthracites are interesting for their abundance, low cost, high carbon content and already existing micropores [1,4– 8]. Physical activation either with CO 2 or with steam has been successfully used for the production of microporous *Corresponding author. Tel.: 133-2-3825-5375; fax: 133-23863-3796. ´ E-mail address: [email protected] (F. Beguin).

activated carbons from anthracites [4–10]. However, this process is hardly applicable at the industrial scale, because very long activation times are necessary due to their low reactivities. To solve this problem, in some processes, anthracite is pretreated before its physical activation. It has been shown that air preoxidation, before the activation, is one of the most efficient possibilities for obtaining good adsorbent materials [8,9]. The preliminary oxidation causes the creation of a primary pore structure which facilitates the activation process [7,10]. Prechlorination has also been used, resulting in increasing the reactivity of nascent carbon sites and in creating additional transport channels for the activating gas into the interior of anthracite [4]. Although the combined chemical–physical activation was successfully used and well documented for the production of activated carbons from non-coal precursors (such as peach stones) [11,12], only a few attempts to use this process for anthracites have been made to date [13– 17]. It has been shown that the treatment of anthracite from La Mure (France) by a solution of perchloric and nitric acids, followed by exfoliation, does not affect the pore

0008-6223 / 02 / $ – see front matter  2002 Published by Elsevier Science Ltd. PII: S0008-6223( 01 )00288-3

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texture, while applied to Hongay anthracite, it essentially changes the pore size distribution of the resulting activated carbon [14]. The authors have suggested that this is due to the different contents of volatile matter, of hydrogen and graphite-like complexes for these two kinds of anthracites. On the other hand, it has been shown that an appropriate chemical modification of common precursors prior to physical activation is a very flexible method for the preparation of activated carbons, ranging from only microporous to only mesoporous [11,12]. For lignocellulosic materials, one of the variables having a large effect on the porosity of the resulting activated carbons is shown to be the degree of impregnation [12]. Previously, we studied the formation of activated carbons from anthracite (La Mure) by using low temperature chemical modification with HClO 4 or Mg(ClO 4 ) 2 prior to physical activation with CO 2 [15–17]. This chemical pretreatment leads to a noticeable increase in the reactivity of the modified anthracite, and results in a better accessibility of the internal surface [15,17]. Furthermore, the reagents inserted in anthracite take part in creating new pores by oxidation reactions, and leave a microporosity upon their subsequent removal during the activation process [16]. The amount and the distribution of the incorporated chemicals govern the evolution of carbon porosity [15]. The objective of this work is to analyse more carefully the relationship between the chemical modification of anthracite with HClO 4 or Mg(ClO 4 ) 2 and the porosity development caused by the physical activation of the products with carbon dioxide.

2. Experimental The experiments were performed with anthracite from La Mure (France). All the samples were demineralised using HCl / HF treatment [17]. In some cases, raw anthracite was pyrolysed in a horizontal bed reactor under nitrogen up to 10008C at a heating rate of 6008C / h and a soaking time of 10 h. The chemical modification of raw and pyrolysed anthracites consisted of reactions with either Mg(ClO 4 ) 2 at 4508C for 1.5 h (anthracite:Mg(ClO 4 ) 2 51:1 or 1:2) [15] or HClO 4 . Several methods were employed to carry out the chemical pretreatment by HClO 4 . The basic one is a step-by-step increase of temperature from 120 to 1608C over 2.5 h and a final plateau at 1608C for 1.5 h [17]. For studying the influence of treatment time and temperature, samples were subjected to a rapid temperature increase up to 1608C followed by holding for 1.5 or 4 h at this temperature. In order to detail the effect of rinsing with water on further activation, some samples were washed with water. After chemical pretreatment, the modified anthracite was filtered and freeze-dried to remove the excess acid and water. All the samples (2 g) were submitted to CO 2 activation (flow-rate 400 ml / min) after rapid heating up to 8508C

(within 10 min, so-called thermal shock) in a tubular furnace, for different soaking times (5–24 h) in order to obtain activated carbons with 10–80% burn-off. In this way, six different series of materials were prepared from: 1. raw anthracite 2. raw anthracite chemically modified by HClO 4 3. raw anthracite chemically modified by HClO 4 and washed with water 4. raw anthracite chemically modified by Mg(ClO 4 ) 2 5. pyrolysed anthracite 6. pyrolysed anthracite chemically modified by HClO 4 Nitrogen adsorption measurements at 77 K were carried out with a Sorptomatic 1900 (Carlo Erba Instruments). Prior to adsorption, the samples were degassed for at least 24 h at 2408C under a pressure of 10 23 Pa. The equivalent surface areas were determined from the adsorption isotherms using the BET equation [18]. The Dubinin–Raduskhevich and B.J.H. methods were applied, respectively, to determine the micro and mesopore volumes [19,20]. The elemental analyses were performed by the Service Central d’Analyse du CNRS (Vernaison, France) following standard procedures. Pretreated anthracite was decomposed by thermogravimetry analysis using a laboratory-made apparatus consisting essentially of a Setaram microbalance type MTB 10-8 connected with a Pyrox oven type VL30 and with a Balzers multichannel mass spectrometer type QMS420 for the analysis of evolving gases. The thermobalance was first evacuated until 5 Pa was reached and then the experiment was carried out under an argon atmosphere with a flow-rate of 1008 / h up to 8508C and a soaking time of 6 h at this temperature. In order to reproduce conditions close to the first 10 min of the activation treatment under CO 2 , all the chemically treated carbons were also subjected to a thermal shock, i.e. rapid heating up to 8508C for 10 min under nitrogen flow, and the weight loss was determined by comparing the mass of resulting material to that of the initial one. The different types of functional groups and incorporated species were identified by spectroscopic methods: X ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared (FTIR) spectroscopy. An Escalab MK2 (VG Instrument) apparatus equipped with an X-ray aluminium source (Ka radiation) and a Nicolet 5SXC spectrometer in the range 4000–400 cm 21 with a resolution of 16 cm 21 , using pressed KBr pellets which contained approximately 0.5 wt.% carbon, were used to realise these measurements, respectively.

3. Results and discussion

3.1. Physical activation: evolution of the pore volume One of the effects of chemical modification by HClO 4 is

S.B. Lyubchik et al. / Carbon 40 (2002) 1287 – 1294

a noticeable reduction of the activation time. The gasification rates of anthracite treated by HClO 4 , with or without water washing, are essentially comparable and greater than those of raw anthracite and anthracite treated by Mg(ClO 4 ) 2 (Table 1). The pyrolysed samples are less reactive than raw anthracite even after chemical modification. This is due to the reorganisation of anthracite towards a lamellar microtexture during pyrolysis, and the acid treatment is weakly efficient. The highest gasification rates are observed for the samples with the lowest carbon content (Table 1), i.e. with the highest amount of incorporated species. This provides proof that the reagent has an important effect on the activation process. There are several linear tendencies between burn-off and BET specific surface area, depending on the type of treatment. The equivalent surface areas range from 400 to 500 m 2 / g at about 30% burn-off to 1200–1600 m 2 / g at 60–70% burn-off (Fig. 1). The chemical modification of raw anthracite has a direct influence on the final pore size distribution. The evolution of the micro / mesopores volume ratio during activation, and the pore size distribution in the resulting activated carbons (at 50% burn-off) may be summarised for the six series as follows (Table 2): 1. Raw anthracite: during activation the micropores are slowly opened; the mesopores are scarcely developed. The resulting activated carbon mainly consists of micropores 2. Raw anthracite1HClO 4 : rapid creation of narrow micropores during the thermal shock step (10 min for reaching 8508C) followed by their destruction and widening during activation. The resulting activated carbon consists of well-balanced micro and mesopores 3. Raw anthracite1HClO 4 1washing: creation of micro and mesopores during activation is slower than for the previous case. The resulting activated carbon is porous and mainly consists of micropores with some amount of mesopores 4. Raw anthracite1Mg(ClO 4 ) 2 : increasing mesoporosity during the activation process, which started to be created during the chemical treatment; the micropores are scarcely developed. The resulting activated carbon

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Fig. 1. Evolution of the BET surface area as a function of burn off for activated carbons from: h, pyrolysed anthracite; j, pyrolysed anthracite1HClO 4 ; d, raw anthracite; m, raw anthracite1HClO 4 ; ., raw anthracite1HClO 4 1washing; 1, raw anthracite1Mg(ClO 4 ) 2 .

is significantly mesoporous, however with a low total pore volume. For 50% burn-off, Vmeso /Vmicro 50.64 and 2.5 for Mg(ClO 4 ) 2 / anthracite ratios 1:1 and 2:1, respectively 5. Pyrolysed anthracite: very low reactivity of the sample during activation. The final product is essentially a non porous carbon 6. Pyrolysed anthracite1HClO 4 : the chemical modification leads to increasing reactivity of the pyrolysed anthracite. There is a slow development of microporosity during activation. The resulting carbon consists of micropores with a lesser amount of mesopores. Such differences during activation are caused by insertion and oxidation reactions through the chemical pretreatment.

3.2. Relation between the chemical nature of the pretreated anthracite and the activation efficiency An amount of chlorine is detected by XPS in all

Table 1 Rate of gasification under CO 2 atmosphere for different sets of precursors Precursors

Gasification rate a (mass % h 21 )

Carbon content before activation, C (%)

Pyrolysed anthracite Pyrolysed anthracite1HClO 4 Raw anthracite Raw anthracite1Mg(ClO 4 ) 2 Raw anthracite1HClO 4 Raw anthracite1HClO 4 1washing

1.81 2.09 2.31 2.27 3.23 3.23

97.6 96.7 90.6 87.6 75.2 80.6

a

Calculated from the slope of the initial part of the weight loss curve.

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Table 2 Equivalent surface area and pore volume of activated carbons Burn-off a (%)

S aBET , (m 2 / g)

a V micro (cm 3 / g) (D/ R)

V ameso (cm 3 / g) (B.J.H)

Vmeso / Vmicro b

Raw anthracite

52.4

865

0.54

0.11

0.18

Raw anthracite1HClO 4

70.0

1600

0.61

0.30

0.49

Raw anthracite1HClO 4 1washing

72.0

1430

0.71

0.16

0.23

Raw anthracite1 Mg(ClO 4 ) 2

54.8

785

0.25

0.16

0.64

Pyrolysed anthracite

43.5

300

0.06

0.07

1.17

Pyrolysed anthracite1 HClO 4

47.6

642

0.35

0.10

0.26

a

Data for 24 h activation with CO 2 at 8508C. b Data for 50% burn-off.

chemically pretreated samples, whatever the time of treatment, temperature and the nature of the chemical agent. The Cl2p core level signal is composed of two contributions, which are attributed to chlorine bound to aromatic carbons (peak centred at |201 eV) and to chlorine in perchloric acid (peak centred at |208 eV) (referred to Au4f 7 / 2 584 eV) [21]. The main difference between the pretreated samples lies in the distribution of these two contributions (Fig. 2). In the case of step-by-step acid treatment, chlorine is mainly bound to carbon with some contribution of chlorine belonging to HClO 4 : the Cl (in HClO 4 ) / Cl (bound to carbon) ratio is 0.5 (Fig. 2a), whereas it is 0.20 in the case of the washed sample (Fig. 2b). The latter trend is the consequence of water dissolution of adsorbed acid. The acid treatment for 1.5 h at 1608C results in the increase of chlorine contribution from HClO 4 : the Cl (in HClO 4 ) / Cl (bound to carbon) ratio is 3 (Fig. 2c). Acid treatment for 4 h at 1608C completely eliminates the chlorine contribution of HClO 4 , showing that HClO 4 was fully reacting with anthracite with time (Fig. 2d). For Mg(ClO 4 ) 2 treated anthracite, chlorine is only bound to carbon. HClO 4 treatment of pyrolysed anthracite allowed only a very low incorporation of chlorine belonging to HClO 4 (Cl / C5 0.002). Due to the lamellar character of this material, the edge plans are poorly accessible to the reagent. The C1s core level signals also showed different behaviour of the chemically modified samples (Fig. 3). After fitting, peaks could be attributed to C–C (284.6 eV), C–O (286.3 eV), C=O (288.9 eV) and O–CO (290.6 eV) bonds. The latter contribution, always very weak, was only observed for some acid treated samples. The presence of C=O bonds is especially remarkable for the 4-h treatment at 1608C (Fig. 3d), confirming a strong oxidation of carbon in this case. On the other hand, the Mg(ClO 4 ) 2 treatment almost does not promote the production of surface oxygen functionality (Fig. 3e), due to a strong combustion of

anthracite as shown by the weight loss after chemical treatment, |18% (d.a.f). For the pyrolysed sample (Fig. 3b), the oxidation is very limited, confirming the fact that the reaction did not take place with this highly organised char. The composition of the various samples was determined after freeze-drying (Table 3). Marked differences were observed for carbon, oxygen and chlorine contents, which should reveal the gasification ability of the modified samples. The highest carbon content is observed for the pyrolysed anthracite, chemically modified or not. In this case, there is a poor oxygen functionality (the O / C atomic ratio is negligible), due to the graphite-like structure of pyrolysed anthracite [22] with few active sites for the reaction with HClO 4 . The low carbon amount after treatment of raw anthracite with HClO 4 has to be attributed both to reagent insertion and to carbon oxidation. After washing, the carbon content is higher than in the other cases, confirming that ‘free’ HClO 4 is eliminated by dissolution into water. Chlorine is present in a consequent proportion in all the acid-treated materials, in good agreement with the XPS data (Fig. 2). The atomic Cl / C ratio ranges from 0.014 to 0.023 depending on time, temperature and washing. In the case of Mg(ClO 4 ) 2 -treated samples there is a lack of chlorine, but Mg is still present even after 24 h activation with CO 2 at 8508C. The amount of water present in the bulk is noticeable despite the freeze-drying. This could be due to a high hygroscopic effect of the incorporated species, especially HClO 4 . The FTIR spectra of acid modified samples confirm the XPS observations. A peak at |1600 cm 21 assigned to the aromatic C=C stretching is especially remarkable for pyrolysed anthracite (Fig. 4a). Following the oxidation treatment with HClO 4 , a well defined band at |1730 cm 21 can be ascribed to C=O bonds (Figs. 4b and 5b). The smaller band observed for pyrolysed anthracite is due to a hindered reactivity related with the lamellar microtexture

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Fig. 2. Cl2p XPS spectra obtained on raw anthracites chemically modified by HClO 4 : (a) step-by-step; (b) step-by-step washed with water; (c) for 1.5 h at 1608C; (d) for 4 h at 1608C.

of this material. The bands at 1230 and 1100 cm 21 could be assigned to asymmetric and symmetric C–O–C stretching, respectively. The formation of all these oxygenated groups may be attributed to redox reactions of incorporated HClO 4 with carbon during the chemical treatment. For a

Fig. 3. C1s XPS spectra obtained on chemically modified anthracites: (a) raw1HClO 4 step-by-step; (b) pyrolysed1HClO 4 stepby-step; (c) raw1HClO 4 for 1.5 h at 1608C; (d) raw1HClO 4 for 4 h at 1608C; (e) raw1Mg(ClO 4 ) 2 .

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Table 3 Composition of raw and modified anthracite C (%)

O (%)

H/C

Cl / C

O/C

H2O / C

Raw anthracite

90.6

3.1

0.24

,0.001

0.025

–

Raw1HClO 4 step-by-step

75.2

13.2

0.29

0.023

0.130

0.055

Raw1HClO 4 for 4 h at 1608C

78.2

12.5

0.26

0.014

0.120

0.035

Raw1HClO 4 for 1.5 h at 1608C

80.6

–

0.23

0.019

–

0.040

Raw1HClO 4 step-by-step 1washing

86.3

0.19

0.016

0.050

0.010

Raw 1Mg(ClO 4 ) 2 a

87.6

0.16

0.004

–

–

Pyrolysed anthracite

97.6

0.3

0.06

,0.001

0.002

–

Pyrolysed 1HClO 4

96.7

1.3

0.07

0.002

0.010

0.005

a

5.8 –

There are 880 ppm of Mg.

Fig. 4. FTIR spectra of pyrolysed anthracite (a) and pyrolysed anthracite1HClO 4 (b).

Fig. 5. FTIR spectra of raw anthracite (a), raw anthracite1HClO 4 (b), raw anthracite1Mg(ClO 4 ) 2 (c).

S.B. Lyubchik et al. / Carbon 40 (2002) 1287 – 1294

comparison, Mg(ClO 4 ) 2 treatment of raw anthracite does not promote the production of an extensive surface oxygen functionality (Fig. 5c). According to TGA analysis, a weight loss of about 1–4% was systematically observed for all the treated samples during vacuum processing at room temperature (Table 4). This could be due to the elimination of adsorbed water. The total weight loss of pretreated anthracite under thermal shock conditions (8508C, N 2 ) is attributed to the thermal desorption of incorporated chemicals and water (Table 4). In all cases, TGA shows that the amount of evolved products increases with the soaking time at 8508C. A comparison of pore texture parameters (Table 2) and of TGA data (Table 4) shows that there is a tight dependence between the porosity development and the weight loss provoked by a thermal shock at 8508C. Hence, the evolved chemicals are able to influence the type of pore structure created during the simultaneous oxidative gasification of modified anthracites in CO 2 atmosphere. Furthermore, as could be detected by mass spectrometry during the thermal ramp on HClO 4 -treated anthracite, there is a release of carbon dioxide, mainly at |200 and 6008C (Fig. 6). The first peak could be ascribed to the low temperature oxidation of carbon by incorporated HClO 4 . This redox process is less important for the washed sample because water treatment eliminates an important part of acid. The second peak could be attributed to the desorption of surface oxygenated groups at high temperature, and appears to be more important for washed samples. The acid treatment for 4 h at 1608C causes the decrease of low temperature outgassing. This is compatible with the XPS analysis that detected almost no free HClO 4 and mainly chlorine involved in bonds with aromatic carbons. As

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Fig. 6. CO 2 evolution for the anthracite modified by HClO 4 step-by-step (1), for the same sample washed by water (2) and for anthracite treated by HClO 4 for 4 h at 1608C (3).

expected, there is no release of carbon dioxide for the Mg(ClO 4 ) 2 -treated sample or for the pyrolysed anthracite treated by HClO 4 . Chemical treatment also modified the microtexture of anthracite. The detailed modification of anthracite after chemical treatment with HClO 4 can be found elsewhere [16,17]. The most important parameters in the chemical pretreatment of anthracite, which had a high effect on the pore volume of the resulting activated carbons (Table 2) were

Table 4 Weight loss (Wt.%) of raw and modified anthracite measured by thermogravimetric analysis (TGA) in different conditions and by thermal shock (8508C, 10 min, under nitrogen) Wt. % After evacuation

After 10 min at 8508C

After 6 h at 8508C

By thermal shock

Raw anthracite1HClO 4 step-by-step

3.50

21.3

22.6

21.3

Raw anthracite1HClO 4 for 4 h at 1608

4.40

22.1

35.3

32.0

Raw anthracite1HClO 4 for 1.5 h at 1608C

3.10

17.3

18.5

15.8

Raw anthracite1HClO 4 step-by-step1washing

2.90

10.6

17.9

12.8

Raw anthracite1 Mg(ClO 4 ) 2

1.70

4.40

5.40

3.57

Pyrolysed anthracite

0.04

0.14

3.50

Insignificant

Pyrolysed anthracite1 HClO 4

0.07

3.70

4.30

3.17

1294

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found to be the treatment time and temperature, the nature of the chemical agents and the anthracite texture. In the case of acid treatment, time and temperature are responsible for differences in HClO 4 introduction and formation of C–Cl bonds. Thus, during the step-by-step process (slow increase of temperature for 4 h) both insertion and oxidation reactions contribute to anthracite modification, resulting in a better accessibility of the activating gas to the interior of the grains and to a noticeable increase of reactivity. As a result, activated carbons with well-balanced micro and mesopores are formed. Decreasing the time of acid treatment for 1.5 h at 1608C causes the deficiency of oxidation reactions, despite a high chlorine introduction (Cl / C50.019). As has been shown elsewhere [16], such a limited treatment does not allow successful production of activated carbon. On the other hand, increasing the time of acid treatment to 4 h at 1608C mainly leads to a strong oxidation (O / C50.120) with almost no insertion of HClO 4 . In this case, during gasification, the particles are rather burnt on their outer part than activated, and the obtained activated carbon is weakly porous. Washing with water after chemical modification with HClO 4 allows to moderate the subsequent activation process, resulting in the production of highly microporous activated carbon, even at a deep burn-off. During the chemical treatment by Mg(ClO) 4 , the reaction of anthracite creates a primary mesoporous structure, which is further enhanced during the activation process, finally giving a mesoporous carbon. The pyrolysis of raw anthracite favours the formation of a graphite-like structure inaccessible to the activating gas, that limits the pores development. Modification of such samples does not promote either sufficient surface oxidation or chemicals incorporation. As a result, pyrolysed anthracite leads to weakly porous activated carbons with poor adsorptive properties.

4. Conclusion FTIR, XPS and TGA data on pretreated anthracite prove the effectiveness of the chemical modification through which the reagents penetrate inside the anthracite microtexture and also react to some extent with carbon. These insertion and oxidation reactions essentially modified the external and internal surfaces of anthracite, and caused different behaviour during physical activation by CO 2 . The extent of surface modification and the contribution of these reactions to the activation process depend on the textural properties of anthracite and on the chemical treatment conditions, i.e. the kind of reagent, the treatment time and temperature. By varying these parameters, activated carbons in a wide range of porosity from mainly microporous to mainly mesoporous have been obtained from anthracite.

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