Porous Carbon Obtained by Carbonization of PET Mixed with Basic

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Ind. Eng. Chem. Res. 2009, 48, 7110–7116

Porous Carbon Obtained by Carbonization of PET Mixed with Basic Magnesium Carbonate: Pore Structure and Pore Creation Mechanism Jacek Przepio´rski,*,† Justyna Karolczyk,† Kazuhiro Takeda,‡ Tomoki Tsumura,‡ Masahiro Toyoda,‡ and Antoni W. Morawski† Institute of Chemical and EnVironmental Engineering, West Pomeranian UniVersity of Technology, ul. Pulaskiego 10, 70-322 Szczecin, Poland, Faculty of Engineering, Oita UniVersity, 700 Dannoharu, Oita 870-1192, Japan

Porous carbon materials were prepared by heating of a PET [poly(ethylene terephtalate)] and magnesium carbonate (3MgCO3 · Mg(OH)2 · 3H2O) mixture under Ar gas flow, followed by acid washing. The inorganic component of the mixture underwent thermal decomposition with formation of MgO and evolution of gaseous products including CO2 and H2O. Influence of the magnesium compound mixed with PET, and carbonization temperature on the pore structure of carbonaceous material is presented. As found, porosity of prepared materials depended on magnesium carbonate loading in the starting mixture and on heating temperature. The impact of CO2 and H2O evolved during heating on the pore structure is discussed, and a mechanism of pore formation is proposed. Introduction Control of pore structure in carbon materials is one of many aspects related to designing of carbon adsorbents. There are a number of potential applications for porous carbons, such as water treatment, requiring adsorption of impurities with a wide range of molecular size. Therefore, adsorbents containing pores of various size are demanded. Lots of methods have been described in literature for preparation of microporous carbons, as well as those abundant in mesopores.1-3 Some inorganic compounds are known4 to create either micropores or mesopores in carbon materials, and one of the potential carbon precursors widely studied is synthetic material that causes environmental problems [PET, poly(ethylene terephtalate)]. As a rule, PETbased activated carbons prepared through the conventional carbonization/activation process show predominant microporous character5 and reveal a low volume of mesopores. To produce carbons with an increased mesopore share, Kartel et al. proposed a new procedure, consisting of carbonization of PET with H2SO4 and successive conventional steam activation.6 Since preparation procedures for porous carbons, especially for bimodal carbons with a high contribution of both micro- and mesopores, usually require carbonization and subsequent activation, this routine can be considered as troublesome. Hence, utilization of suitable agents potentially capable to generate pores of various sizes in carbon material in one-step process appears to be reasonable. A simple novel technique for preparation carbons revealing predominant share of mesopores was proposed by Inagaki et al.7-9 Mesoporous carbons were prepared by heating of carbon precursor mixed with MgO or with some thermally unstable organic magnesium compounds, such as acetate, citrate, or gluconate, under an inert atmosphere, followed by dissolution of MgO particles formed in the carbon matrix. Because mesopores formed were remained pores after dissolution of the MgO nanoparticles, bimodal carbons with an increased contribution of two different pore diameters were easily obtained by * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +48 914494686. Tel. +48 914494326. † West Pomeranian University of Technology. ‡ Oita University.

mixing of two different MgO precursors into carbon precursor.10 According to the authors, the preparation process is accompanied by the thermal decomposition of magnesium acetate with evolution of gases leaving heated mixture, whereas both gluconate and citrate groups became additional sources of carbon. In contrast to the conventional activation processes, no chemical interaction was detected between MgO and carbonaceous product formed from the carbon precursor during heating.11 Moreover, no influence of gaseous products evolved from the MgO precursors on pore structure of the carbonaceous product was observed. In the present paper, basic magnesium carbonate (BMC) [3MgCO3 · Mg(OH)2 · 3H2O] was used as the pore creating agent because this chemical undergoes three-step thermal decomposition leaving MgO as a solid residue and evolving large volume, 7 mol/mol BMC, of gases including CO2 and H2O12 3MgCO3 · Mg(OH)2 · 3H2O f 3MgCO3 · Mg(OH)2 + 3H2O (210 - 320 °C) (1)

3MgCO3 · Mg(OH)2 f 3MgCO3 + MgO + H2O (340-490 °C) 3MgCO3 f 3MgO + 3CO2 (490 - 550°C)

(2) (3)

The gases are commonly used as physical activators. Thus, in addition to the outcome of MgO presence, heating of PET mixed with BMC may be accompanied by the reaction of CO2 and H2O with carbon material formed from PET. For that reason, two proceeding simultaneously ways of pore generation may be expected. As a consequence, generation of pores showing different sizes, dependent on the way of their creation, seems to be reasonable to take into consideration. Moreover, utilization of BMC should avoid high burn off of carbon material, normally taking place during the conventional activation process,13 when excessive amounts of the physical activators are used. Experimental Section Materials. BMC and MgO were of reagent grade purchased from POCH S. A., Poland. PET was of commercial grade,

10.1021/ie801694t CCC: $40.75  2009 American Chemical Society Published on Web 06/24/2009

Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009

produced by Elana S. A., Poland. High purity (99.999%) argon gas purchased from Messer (Austria) was used during preparations. Preparations. In this study, several preparation steps were applied to obtain porous carbons. As first, BMC was mixed with PET grains at three BMC/PET weight ratios: 30:70, 50:50, and 70:30. Next, to melt PET contained in the mixture, the mixture was heated in air at 265 °C. After it was cooled down, the solid mass obtained was powdered and again subjected to the fusing/ powdering cycle. Subsequently, 500 mg of the powdered BMC/ PET mixture obtained by this manner was treated by means of heating (10 °C/min) up to 550, 700, or 850 °C in Ar atmosphere (25 cm3/min), and the final temperature was maintained for 1 h. After they were cooled, the samples were ground in an agate mill. To isolate carbon material, the powdered products were washed with acid (10% HCl, 400 cm3), rinsed with 400 cm3 of distilled water, and then filtered. Finally, the obtained cakes were dried at 105 °C for 12 h in air. For reference, carbon materials from PET loaded with magnesium oxide were also prepared. The preparation procedure utilized here was the same as used for the carbon obtained from PET loaded with BMC. Methods. X’Pert PRO Philips diffractometer using Cu KR (λ ) 1.54056 nm) radiation was used to obtain X-ray diffraction (XRD) patterns. The nitrogen adsorption and desorption isotherms at 77 K were measured using Autosorb 3 (Quantachrome, U.S.A.) equipment. Prior to the adsorption measurements the samples were degassed for 24 h at 300 °C under high vacuum. Specific surface area was determined by the BET method. Micropore volume, micropore surface area, external surface area, and total surface area were estimated by Rs method. Here the adsorption isotherm of N2 on carbon black was adopted as the standard isotherm. The pore size distribution curves were obtained from the adsorption branches of the nitrogen adsorption-desorption isotherms using the Barrett-Joyner-Halenda (BJH) method. The BJH method was also employed for determining cumulative pore volume plots. Thermogravimetric analyses were carried out on STA 449 C TGA apparatus (Netzsch, Germany) in Ar atmosphere (25 mL/min), and heating rates were 10 °C/min. The temperature programmed desorption (TPD) measurements were carried out with use of thermal desorption spectrometer (TDS1200 apparatus, ESCO Ltd., Japan) equipped with a quadrupole mass spectrometer. The procedure employed consisted of heating samples from room temperature to 1000 at 60 °C/min and under working pressure of ∼10-7 Pa. The evolution of four gases, including hydrogen, water, carbon oxide, and carbon dioxide, was continuously monitored during all the TPD runs. X-ray photoelectron spectroscopy (XPS) analyses were performed with a Prevac system equipped with Scienta SES 2002 electron energy analyzer. The spectra were recorded with the Mg KR radiation and pass energy of the electron energy analyzer was set at constant mode, 50 eV. The binding energy scale was referred to the highest energy of C1s component found after deconvolution of the spectra and set to be 284.6 eV. The powdered samples glued to a carbon conductive disks were mounted on the sample carrier and were kept under high vacuum overnight before XPS analysis. Binding energy corrections, background subtraction and deconvolution of the spectra were performed with use of CasaXPS software. Results XRD analysis. Examples of XRD patterns shown in Figure 1 clearly indicate existence of MgO in the products obtained after treatment of the BMC/PET mixtures. The same phase was found to be the only solid product formed during heating of

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Figure 1. XRD patterns of 3MgCO3 · Mg(OH)2 · 3H2O/PET (50:50) mixture after heating at 550 and 700 °C. Table 1. Mean MgO Crystallite Sizes (Calculated from Scherrer Equation, Based on (220) Plane) in BMC/PET and MgO/PET Mixtures Heated at Various Temperatures BMC/PET, 50:50 treatment temperature [°C]

mean crystallite size [nm]

550 700 850

9 9 11

MgO/PET, 50:50 treatment temperature [°C]

mean crystallite size [nm]

550 850

19 19

pure BMC. For that reason it was assumed that the heat-treated materials, irrespective of the ratio of components in the starting material and temperature in the range used in this study (550-850 °C), contained MgO. This remains in agreement with results of Helou12 that reported complete decomposition of BMC at temperature up to ∼550 °C with formation of MgO as a solid residue. As expected, magnesium oxide was also detected in the materials remained after treatment of the MgO/PET mixtures at 550, 700, and 850 °C. The mean MgO crystallite sizes calculated from Scherrer formula are included in Table 1. Irrespective of the treatment temperature, the mean size of MgO crystallites in materials obtained from the BMC/PET mixture was about two times smaller than in MgO/PET-based products. The size of MgO crystallites in the latter was comparable to that (18 nm) of original MgO used for preparations. Nitrogen Adsorption/Desorption (77 K) Isotherms Analysis. Figure 2 contains nitrogen adsorption/desorption isotherms (77 K) on the prepared carbons as a function of BMC/ PET loading ratio in raw materials (at the same treatment temperature, 850 °C) and as a function of treatment temperature (at the same BMC/PET ratio, 50:50). Isotherms for MgO/PETbased samples are presented for reference. The pore parameters calculated for the obtained materials differ from one to another and are influenced by the quantitative and qualitative compositions of the starting mixtures as well as by the final treatment temperature (Table 2). Influence of BMC Loading on the Pore Parameters of Obtained Carbons. As can be clearly seen in Figure 2, shapes of all the nitrogen adsorption/desorption isotherms measured at 77 K are, according to Brunauer, Deming, Deming, and Teller (BDDT) classification,14 of type IV and show a hysteresis indicating presence of slit-shaped mesopores in the prepared materials. The isotherms also reveal an underlying type I isotherm with a considerable adsorption at low relative pressures, which is an indication of microporosity. A strong growing tendency in nitrogen adsorption can be observed with the BMC loading in the starting mixture with PET. Hence, among samples

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Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009 Table 2. Influence of the Components’ Ratio in the Starting Mixture and Final Treatment Temperature on Pore Parameters of the Prepared Carbons, from N2 Adsorption Isotherms at 77 K by BET and rs Plot Methods Rs analysis SBET (m2/g)

Stotal (m2/g)

Sext (m2/g)

Smicro (m2/g)

Vmicro (cm3/g)

470 284 677 632 1505

618 235 473 503 557

0.20 0.10 0.15 0.17 0.16

279 332 343

695 584 573

0.23 0.20 0.19

BMC/PET 30:70 50:50 50:50 50:50 70:30

(13.2 MgO)a, 850 °C (22 MgO)a, 550 °C (22 MgO)a, 700 °C (22 MgO)a, 850 °C (30.8 MgO)a, 850 °C

944 531 1064 1049 1984

1088 519 1150 1135 2063

MgO/PET 30:70 (30 MgO) 700 °C 50:50, (50 MgO) 700 °C 50:50 (50 MgO) 850 °C

818 799 788

974 915 916

a Values in parentheses are weight ratios of MgO conversion for BMC.

Figure 2. N2 adsorption/desorption isotherms (77 K) on carbon materials: (a) obtained from BMC/PET mixtures, heated at 850 °C, influence of components contents in raw material, (b) obtained from BMC/PET mixture (50:50), influence of the heating temperature, and (c) obtained from MgO/ PET mixtures, influence of the heating temperature and components loadings.

treated at 850 °C, the highest N2 uptake reveals sample obtained from the raw mixture loaded with the largest amount (70:30) of the salt. Consequently, both BET and mesopore areas markedly depend on the BMC percentage (and thus on MgO, the product of BMC decomposition, content) in starting mixture. The carbon materials obtained from BMC/PET at 850 °C exhibit high BET areas ranging from ∼950 to nearly 2000 m2/g as well as high external surface area from 470 to ca. 1500 m2/g (Tab. 2). This remains in agreement with results of others, dealing with carbonization of MgO/PET system.8 However, despite the considerable increase in the total and external areas, there is a slight decrease in micropore area (and volume) with BMC shares in the starting mixtures. In general, samples prepared from BMC/PET reveal higher nitrogen uptake (Figure 2a and b) than MgO/PET-based ones (Figure 2c), even MgO content in the latter is much higher (Table 2). Moreover, contrary to the carbon materials obtained from BMC/PET mixtures, no strong depen-

dence of the components ratio in raw mixtures on nitrogen uptake by MgO/PET-based carbons can be noticed. The observations given above indicate different action of MgO and BMC on PET during heating. Influence of Treatment Temperature on Pore Parameters of Obtained Carbons. The isotherms measured for samples heated at various final temperatures are presented in Figure 2b. Increasing the treatment temperature from 550 to 700 °C results in a substantial increase in N2 uptake (and consequently in BET area, micropore area and volume, and mesopore area, Table 2). However, further increase in the heat-treatment temperature to 850 °C is reflected by weakened difference in the shape of N2 adsorption isotherms. In a consequence, the total surface area calculated for BMC/PET-based carbon prepared at 850 °C is comparable to that obtained for the carbon prepared at 700 °C. This indicates proceeding of some processes in the treated mixture that may proceed at temperatures above 550 °C. Taking into account other pore parameters, certain increase in the micropore volume and area jointly with decrease in the external area can be noticed as a consequence of an increase in the treatment temperature from 700 to 850 °C. This is opposite to MgO/PET-based carbons revealing only slightly lower micropore area and volume together with a little higher external area, as result of the increase in treatment temperature from 700 to 850 °C. Pore Size Distributions and Cumulative Pore Volumes. As illustrated in Figure 3, porous carbons prepared from BMC/ PET mixtures exhibit characteristic, different than that of carbons prepared from PET loaded with MgO, pore size distribution in the mesopore region. As shown in Figure 3a and b, mesopore volume in BMC/PET-based carbons remains in a strong relation with both BMC content in the raw material and treatment temperature. Thus, pore volume in the mesopore range increases considerably along with BMC loading in the raw material. This particularly concerns mesopores ranging from 5 to 12 nm in the width. In addition, the volume of mesopores in this width range remains in a noticeable dependence with the preparation temperature. In fact, the carbon obtained at 550 °C reveals low pore volume, without noticeable maxima in the range. However, an outcome of heating of the BMC/PET mixture at higher temperatures, 700 and 850 °C, is a product showing sharply increased pore volume in the range. The differences described above imply temperature-dependent action of either BMC or products of its decomposition on the PET during heating.


Figure 3. Pore size distributions (by BJH method) for carbons obtained (a) from BMC/PET mixtures at various ratios (at 850 °C), (b) at different temperatures (at the same BMC/PET ratio, 50:50), and (c) from PET loaded with MgO.

On the other hand, the carbons prepared from PET mixed with MgO demonstrate predominant share of larger mesopores (20-40 nm in width) and very little contribution of the smaller ones (5-12 nm in width), Figure 3c. Additionally, pore size distributions in the mesopore range are only slightly influenced by the MgO/PET ratio in the raw mixture and no significant influence of the treatment temperature on the pore volume can be observed. Consequently, considering cumulative pore volumes (Figure 4) calculated for BMC/PET and MgO/PET-based carbons, following observations can be drawn: • For comparable MgO loadings (Table 1) and at treatments temperatures 700 and 850 °C, carbons obtained from MgO/ PET mixtures reveal much lower pore volumes than those obtained from BMC/PET. • There is a little increase in the pore volume for MgO/PETbased carbons with MgO loading, and treatment temperature has almost no influence.

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• Pore volume calculated for the carbons obtained from BMC/ PET mixture significantly increases with BMC (and calculated MgO) loading, as well as with treatment temperature. The results presented in this section evidently confirm different action of BMC and MgO on treated PET. It is apparent that the difference results from the action of gaseous products of BMC decomposition, appearing in the treated BMC/PET mixture during heating. Thermogravimetric (TG) Studies. Results of the thermogravimetric measurements are presented in Figure 5. Pure BMC shows three-step decomposition that completes at ∼540 °C. Decomposition of pure PET proceeds rapidly at temperatures from ∼400 °C up to ∼480 °C, followed by a slow mass loss at temperatures up to ∼600 °C. Compared to TG curves collected for the pure components, the thermogram measured for the BMC/PET mixture shows an unexpected progress. Thus, the rapid mass loss starts at ∼380 °C and completes at ∼620 °C, which is higher than that observed for the pure components of the mixture. In addition, further increase in the temperature is accompanied by a slower, however, continuous and distinct mass loss. Because decomposition of BMC completes at temperatures below 620 °C and mass change resulting from PET carbonization above this temperature is negligible, it can be stated that the loss is related to the reaction between gases generated from BMC (CO2 and H2O) and the char formed from PET during heating. TPD Results. Signals for mass/charge (m/z) ratios of 2, 18, 28, and 44 were monitored to examine evolution of H2, H2O, CO, and CO2, respectively. TPD runs were carried out for materials obtained through heating BMC/PET mixture and also MgO/PET mixture as a reference. In each case, samples subjected to TPD measurements were as prepared and not subjected to acid washing. Obtained courses are presented in Figure 6. The patterns obtained during TPD from the BMC/ PET-based sample confirm an intensive evolution of water and carbon dioxide at temperatures from 60 °C up to ∼300 °C. The intensities of CO2 and H2O signals tend to decrease gradually at temperatures from 300 °C up to ∼800 °C and are very low at above 800 °C. Furthermore, initially weak CO and H2 signals start to increase markedly along with temperature, with effect from ∼600 °C. The courses of CO2, H2O, CO, and H2 traces registered during TPD from the MgO/PET-based sample are different. Thus, signals from CO2 and CO are of very low intensity throughout whole the temperature range of the TPD analysis. Nevertheless, some minor evolution of water and hydrogen from the sample can be noticed at temperatures up to ∼250 °C and from ∼600 °C, respectively. XPS Analysis. XPS analysis was used to determine surface oxygen functionalities on studied carbons. The C1s signal for both MgO/PET- and BMC/PET-based samples shows an asymmetric tailing (Figure 7), which is the result of the presence of oxygen functional groups. The functions considered for curve fitting of the studied peaks and binding energies (BE) adopted15-18 in C1s fits are included in Table 3. Fractions of graphitic carbon and carbon bounded with oxygen in individual groups and in CO2, calculated from deconvoluted spectra, are also given in Table 3. The XPS data obtained for BMC/PET-based material indicate a little higher, compared to MgO/PET-based material, content of oxygen surface groups. Among the oxygen complexes considered in both materials, contribution of C-O groups is predominant and fractions of the others are lower. Moreover,

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Figure 4. Cumulative pore volume plots (calculated by BJH method) for carbons obtained from BMC/PET and MgO/PET mixtures.

Figure 7. Deconvolution of the C1s peaks in XPS spectra: (a) BMC/PETbased material and (b) MgO/PET-based material. Figure 5. TG curves for pure BMC, pure PET, and for BMC/PET mixture.

Figure 6. Evolution profiles of H2, H2O, CO, and CO2, during TPD from (a) BMC/PET-based sample after heating up to 700 °C and (b) MgO/PETbased sample after heating up to 700 °C.

BMC/PET-based sample reveals little higher content of carbon from adsorbed CO2. Discussion Because BMC included in the BMC/PET mixture undergoes thermal decomposition, it may be assumed that the external areas calculated for carbon samples (Tab. 2) increase with the MgO content in treated samples. Nevertheless, taking into account pore parameters calculated for BMC/PET-based samples treated at different temperatures and obtained from MgO/PET mixtures, certain discrepancies can be easily noticed. Hence, despite higher MgO contents, samples prepared from MgO/PET (50:50, 700 and 850 °C) reveal much lower total and mesopore areas than

carbons obtained from BMC/PET (22 MgO/50) (700 and 850 °C). Additionally, while higher BMC loadings in BMC/PET mixtures are evidently reflected by higher total and external areas, the effect of MgO loading in MgO/PET mixtures on the areas is minor. So, the porosity of obtained carbons is strongly affected by the qualitative compositions of starting mixtures. Moreover, in the case of BMC/PET-based carbons, the porosity is influenced by the relative contents of BMC and PET in starting mixture as well as by the treatment temperature. This statement is undoubtedly supported by the pore size distributions, especially for mesopores with width ranging from 5 to 12 nm. The volume of mesopores from this range increases with BMC loading in the raw BMC/PET mixture and with the treatment temperature (Figure 3). The important finding is that compared to BMC/PET-based carbons prepared at 700 and 850 °C, the sample prepared at 550 °C shows very low volume of mesopores in the range. Besides, the low mesopore volumes reveal also MgO/PET-based samples independently on the preparation temperature. These results indicate different action of MgO and BMC on PET during heating. Because the range covers mean size of MgO crystallites in BMC/PET after heating (Table 1), one may assume increased volume of these pores as result of MgO presence. However, such explanation do not find confirmation if parameters of MgO/PET-based materials are considered. In fact, the mean size of MgO crystallites (19 nm) contained in the heat-treated MgO/PET mixture is not reflected by an increased volume of the related pores. Hence, an influence of CO2 and H2O (evolved during heating from BMC only) on the porosity of carbonaceous products is apparent. Accordingly, action of the gases on the char formed from PET must be a reason why pore parameters calculated for carbons prepared from BMC/PET and MgO/PET mixtures, are different. Never-


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Table 3. Deconvolution Results of XPS C1s Spectra of Studied Samples C1s peak position, eV sample

284.6 graphitic carbon

286.0 carbon from C-O (phenolic, etheric) groups

287.2 carbon from CdO (carbonyl or quinone) groups

288.7 carbon from O-CdO (carboxyl or ester) groups

290.2 carbon from adsorbed CO2

BMC/PET MgO/PET

72.0 78.0

12.9 10.5

6.0 4.8

3.4 2.9

5.7 3.8

theless, considerable porosity of MgO/PET-based samples suggests that despite the evident influence of CO2 and H2O on porosity of the BMC/PET-based carbon, the increased pore volume, to some extent, may be also caused by MgO presence. This is especially related to the mesopores with 5-12 nm in width, which range corresponds to the mean size (9-11 nm, Tab. 1) of MgO crystallites formed from BMC mixed with PET. This statement is also supported by results reported by others,8-11 reporting magnesium oxide as an agent capable to form mesopores in a carbon material. Considering two processes that may possibly proceed simultaneously during heating of the BMC/PET mixture, that is, carbonization of PET accompanied by the formation of a rigid structure (char), and BMC decomposition, some phenomena can be taken into account. One is that the gaseous products evolved during BMC decomposition create in the formed char either open pores (and leave the treated material without reaction) or closed pores (and thus remain in them unreacted). It is quite reasonable to assume that the formed pores may be of various sizes and this mechanism can be applied only to the lower treatment temperatures, up. to ca. 600 °C, which is too low for activation of the formed char by the gases evolved from BMC. Because contents of oxygen surface groups detected in carbons by XPS are comparable (Table 3), and the functionalities are stable at temperatures below 200 °C,19,20 the intensive liberation of CO2 and H2O during TPD at temperatures as low as 60 °C (Figure 6a) must be connected with the free gases occluded in the char formed from the BMC/PET-based sample only. This proves the hypothesis that during heating of the BMC/PET mixture gases formed from BMC are entrapped in the char formed from PET. Another possibility is that the gases trapped in the closed pores are kept in the char until temperature reaches point, ∼700 °C, at which the gasification of the char by CO2 and H2O according to the reactions C + H2O f CO + H2 C + CO2 f 2CO starts to be intensive. Thus, some effects may be expected as result of the carbon gasification, that is, opening or widening of the closed pores or creating new ones. If this hypothesis is true, the products of carbon gasification must be formed and certain carbon loss must occur during heating of BMC/PET mixture at temperatures high enough to gasify carbon. Additionally, these effects must be accompanied by appropriate changes in the pore structure of carbonaceous product. As shown in Figure 6a, heating of BMC/PET-based sample above 600 °C is accompanied by an intensive releasing of hydrogen and carbon monoxide. But because the evolution of CO may be due either thermal decomposition of oxygen surface groups20 or carbon gasification according to the reactions given above, the latter possibility can not be surely confirmed. However, as shown in Table 3, the contents of surface complexes undergoing decomposition with evolution of CO, that is, carbonyl, phenolic, and etheric groups, in both studied samples are comparable. Hence, if the occluded CO2 and H2O do not react with carbon,

no hydrogen should be detected during TPD from both considered materials and amounts of carbon monoxide evolved should be comparable. Nevertheless, the TPD profiles in Figure 6 confirm intensive liberation of H2 and CO at above 600 °C from BMC/PET-based material only. Moreover, as indicated on the TG-gram (Figure 5), generation of these gases is accompanied by a distinct loss of carbon. These results testify gasification of the char by the occluded CO2 and H2O. It should be pointed out that noticeable m/z ) 2 and 28 signals were registered during TPD from BMC/PET-based sample also below 600 °C. Certainly, some small amounts of CO and H2 were formed according to the mechanism proposed above already during preparation of the material for the analysis (700 °C). Consequently, the gases were occluded in the formed char together with CO2 and H2O formed from BMC, and finally released during TPD. Because carbon products obtained from BMC/PET at 700 and 850 °C reveal increased, compared to the material prepared at 550 °C, cumulative pore volume (Figure 4), the gasification process discussed above can be considered as the physical activation with a preferential formation of small mesopores, from 5 to 12 nm in the width. On the other hand, there are only traces of CO2 and CO evolved from the MgO/PET-based sample during TPD run (Figure 6b) and amounts of water and hydrogen evolved are negligible. The slightly increased m/z ) 18 signal observed at temperatures up to ∼200 °C corresponds to the evaporation of water contained in the sample and the weak m/z ) 2 signal seen at temperatures above 600 °C is due to the fragmentation of PET decomposition products.21 Hence, the gasification of the char formed during heating of MgO mixed with PET does not arise. In a consequence, independently on preparation temperature, the cumulative pore volume calculated for carbons obtained from the PET mixed with MgO is relatively low. Conclusions Simple heat treatment of the PET loaded with BMC produces porous carbon with a predominant share of mesopores but also with a large contribution of micropores. Application of the decomposable magnesium salt results in obtaining highly porous carbon with BET surface area up to ∼2000 m2/g. The gaseous products of BMC decomposition gasify the char formed from PET and thus play a role of activating agents. Pore volume of carbons prepared according to the described procedure depends on the BMC loading in PET as well as on the temperature utilized during preparations. Hence, to some extent, the porosity of carbons produced can be controlled. Because contribution of mesopores and micropores in the total porosity is considerable, the obtained porous carbons are potentially capable to adsorb molecules of various sizes, like for example phenols and humic acids. Therefore, this feature can be considered as an important advantage in specific applications such as water treatment. Acknowledgment This work was partly supported by the Polish Ministry of Science and Higher Education, Grant No. N N205 012734. The

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authors acknowledge ESCO Ltd., Japan, for TPD measurements. Dr. Dariusz Moszyn˜ski at West Pomeranian University of Technology is acknowledged for XPS analysis. Literature Cited (1) Vinu, A.; Srinivasu, P.; Takahashi, M.; Mori, T.; Balasubramanian, V. V.; Ariga, K. Controlling the Textural Parameters of Mesoporous Carbon Materials. Microporous Mesoporous Mater. 2007, 100, 20. (2) Kyotani, T. Control of Pore Structure in Carbon. Carbon 2000, 38, 269. (3) Dias, J. M.; Alvim-Ferraz, M. C. M.; Almeida, M. F.; Rivera-Utrilla, J.; Sanchez-Polo, M. Waste Materials for Activated Carbon Preparation and its Use in Aqueous-Phase Treatment: A Review. J. EnViron. Manage. 2007, 85, 833. (4) Tamai, H.; Kakii, T.; Hirota, Y.; Kumamoto, T.; Yasuda, H. Synthesis of Extremely Large Mesoporous Activated Carbon and its Unique Adsorption for Giant Molecules. Chem. Mater. 1996, 8, 454. (5) Parra, J. B.; Ania, C. O.; Arenillas, A.; Rubiera, F.; Pis, J. J. High Value Carbon Materials from PET Recycling Materials. Appl. Surf. Sci. 2004, 238, 304. (6) Kartel, M. T.; Sych, N. V.; Tsyba, M. M.; Strelko, V. V. Preparation of Porous Carbons by Chemical Activation of Polyethyleneterephthalate. Carbon 2006, 44, 1019. (7) Inagaki, M.; Kobayashi, S.; Kojin, F.; Tanaka, N.; Morishita, T.; Tryba, B. Pore Structure of Carbons Coated on Ceramic Particles. Carbon 2004, 42, 3153. (8) Morishita, T.; Soneda, Y.; Tsumura, T.; Inagaki, M. Preparation of Porous Carbons from Thermoplastic Precursors and Their Performance for Electric Double Layer Capacitors. Carbon 2006, 44, 2360. (9) Inagaki, M.; Kato, M.; Morishita, T.; Morita, K.; Mizuuchi, K. Direct Preparation of Mesoporous Carbon from a Coal Tar Pitch. Carbon 2007, 45, 1121. (10) Morishita, T.; Ishihara, K.; Kato, M.; Inagaki, M. Preparation of a Carbon with a 2 nm Pore Size and of a Carbon with a Bi-Modal Pore Size Distribution. Carbon 2007, 45, 209.

(11) Morishita, T.; Susuki, T.; Nishikawa, T.; Tsumura, T.; Inagaki, M. Preparation of Porous Carbons by the Carbonization of the Mixtures of the Thermoplastic Precursors with MgO. Tanso 2005, 219, 226. (12) Helou, G.; Tariq, S. A. The Pyrolysis of Basic Magnesium Carbonate Trihydrate. Thermochim. Acta 1993, 228, 123. (13) Laszlo, K.; Tombacz, E.; Josepovits, K. Effect of Activation on the Surface Chemistry of Carbons from Polymer Precursors. Carbon 2001, 39, 1217. (14) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (15) Biniak, S.; Szyman˜ski, G.; Siedlewski, J.; S´wia¸tkowski, A. The Characterization of Activated Carbons with Oxygen and Nitrogen Surface Groups. Carbon 1997, 35, 1799. (16) Polovina, M.; Babic´, B.; Kaluderovic´, B.; Dekanski, A. Surface Characterization of Oxidized Activated Carbon Cloth. Carbon 1997, 35, 1047. (17) Puziy, A. M.; Poddubnaya, O. I.; Socha, R. P.; Gurgul, J.; Wisniewski, M. XPS and NMR Studies of Phosphoric Acid Activated Carbons. Carbon 2008, 46, 2113. (18) Lee, W. H.; Lee, J. G.; Reucroft, P. J. XPS Study of Carbon Fiber Surfaces Treated by Thermal Oxidation in a Gas Mixture of O2/O2 + N2. Appl. Surf. Asci. 2001, 171, 136. (19) Rodriguez-Reinoso, F.; Molina-Sabio, M. Textural and Chemical Characterization of Microporous Carbons. AdV. Colloid Interface Sci. 1998, 76-77, 271. (20) Szyman´ski, G. S.; Karpin´ski, Z.; Biniak, S.; S´wia¸tkowski, A. The Effect of the Gradual Thermal Decomposition of Surface Oxygen Species on the Chemical and Catalytic Properties of Oxidized Activated Carbon. Carbon 2002, 40, 2627. (21) Sanchez, M. E.; Moran, A.; Escapa, A.; Calvo, L. F.; Martinez, O. Simultaneous Thermogravimetric and Mass Spectrometric Analysis of the Pyrolysis of Municipal Solid Wastes and Polyethylene Terephtalate. J. Therm. Anal. Cal. 2007, 90, 209.

ReceiVed for reView November 7, 2008 ReVised manuscript receiVed May 19, 2009 Accepted June 3, 2009 IE801694T

Porous Carbon Obtained by Carbonization of PET Mixed with Basic

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