Ind. Eng. Chem. Res. 1999, 38, 2947-2953
2947
MATERIALS AND INTERFACES High-Porosity Carbons Prepared from Bituminous Coal with Potassium Hydroxide Activation Hsisheng Teng* and Li-Yeh Hsu Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan
Porous carbons with a very high porosity were prepared from an Australian bituminous coal with KOH activation. The preparation process consisted of KOH impregnation followed by carbonization in nitrogen at 500-1000 °C for 0-3 h. Surface area and pore volume of the resulting carbons were found to increase with the carbonization temperature to a maximum at 800 °C and then begin to decrease. It has been suggested that carbon gasification by the released CO2 and the oxygen in potassium containing compounds plays an important role in determining the pore structures. The porosity development was affected by the chemical ratio of KOH to coal. The optimum chemical ratio for preparing a high-porosity carbon varies with carbonization conditions. A process consisting of impregnation at a chemical ratio of 4.25 followed by carbonization at 800 °C for 1 h was recommended for producing a high surface area (>3000 m2/g) carbon. Introduction
Table 1. Analysis of Black Water (BW) Bituminous Coal
Porous carbons, which have a collective name of activated carbon, are prepared by physical activation, gasification of a char in oxidizing gases, or by chemical activation, carbonization of carbonaceous materials impregnated with chemical reagents.1 To allow large amounts of adsorption, these carbons with a high surface area and porosity are desirable. In addition to serving as an adsorbent, high-porosity carbons have recently been used in the manufacturing of highperformance double-layer capacitors.2-5 Generally, the type of porosity of carbons is determined by the type of raw material and the method of activation. Because of the availability and abundance, coal is the most commonly used precursor for activated carbon production.6-8 In physical activation, bituminous coals are preferred since their products are more durable than other coalbased carbons.9 However, lignocellulosic materials are favored in chemical activation,10 especially using H3PO4 as the chemical reagent. It is believed that in biomass activation H3PO4 accelerates the cleavage of bonds between biopolymers, followed by recombination reactions to form a rigid cross-linked structure.11,12 Thus, the response to H3PO4 activation decreases with the degree of coalification.13,14 This has been evidenced by our previous study,15 showing that activated carbons prepared from bituminous coal with H3PO4 activation exhibit only moderate levels of surface area and porosity. Because of the difference in the chemical activation mechanism,16 one study17 has reported that preparation of activated carbons with KOH activation produces a * To whom correspondence should be addressed. Tel.: 886-6-2385371. Fax: 886-6-2344496. E-mail: hteng@ mail.ncku.edu.tw.
ultimate (wt %, dry-ash-free basis) carbon nitrogen hydrogen oxygen sulfur
83.2 3.7 5.1 7.4 0.6
proximate (wt %, as-received) moisture volatile matter fixed carbon ash
2.7 26.7 61.8 8.9
large porosity development in the case of using bituminous coals as the precursor. Thus, the influence of the preparation conditions on the porosity development of the resulting carbons from bituminous coals merits a thorough investigation and study. The present work is devoted to investigating the effects of preparation parameters on the properties of high-porosity carbons from an Australian bituminous coal with KOH activation. The optimum conditions to produce high porosity carbons will be recommended in this paper. Experimental Section An Australian bituminous coal, Black Water (BW, high volatile A in ASTM classification) was used as the starting material. The proximate and ultimate analyses of the raw coal are shown in Table 1. The as-received coal was ground and sieved to 420-1000 µm, before being treated. Chemical activation of the coal was performed by using KOH as the chemical reagent. The activation process was initiated by mixing, in a 250-mL glassstoppered flask, 1 g of the as-received coal with a KOH solution that contained 50 g of water. The flask was immersed in a constant-temperature shaker bath, with a shaker speed of 100 rpm. The mixing was performed at 85 °C and lasted for 3 h. The concentration of the KOH solution was adjusted to give a ratio of chemical reagent to coal (i.e., a chemical ratio) varying in a range 0-8.
10.1021/ie990101+ CCC: $18.00 © 1999 American Chemical Society Published on Web 07/13/1999
2948 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999
After mixing, the coal slurry was subjected to vacuumdrying at 110 °C for 24 h. The resulting chemical-loaded samples were then carbonized in a horizontal cylindrical furnace (60-mm i.d.) in an N2 atmosphere, with a flow of 100 mL/min. Carbonization was carried out by heating the samples at 30 °C/min from room temperature to carbonization temperatures in a range of 5001000 °C, followed by holding the samples at the carbonization temperature for different lengths of carbonization time (0-3 h) before cooling under N2. After cooling, the carbonized products were washed by stirring with 250 mL of 0.5 N HCl solution at 85 °C for 30 min, followed by filtration. The acid-washed sample was then leached by mixing with 250 mL of distilled water at 85 °C several times, until the pH value of the filtrate was above 6. The leached products were then dried by vacuum at 110 °C for 24 h, to give the final carbon products. Specific surface areas and pore volumes of the activated samples were determined by gas adsorption. An automated adsorption apparatus (Micromeritics, ASAP 2010) was employed for these measurements. Adsorption of N2, as the probe species, was performed at -196 °C. Before any such analysis, the sample was degassed at 300 °C in a vacuum at about 10-3 Torr. Nitrogen surface areas and micropore volumes of the samples were determined from the application of the BET and Dubinin-Radushkevich (D-R) equations, respectively, to the adsorption isotherms at relative pressures between 0.06 and 0.2. The amount of N2 adsorbed at pressures near unity corresponds to the total amount adsorbed at both micropores and mesopores; and, consequently, the subtraction of the micropore volume (from the D-R equation) from the total amount (determined at p/p0 ) 0.98 in this case) will provide the volume of the mesopore.18 The average pore diameter can be determined according to the surface area and total pore volume (the sum of the micropore and mesopore volumes), if the pores are assumed to be parallel and cylindrical. Results and Discussion As indicated in the preceding section, the preparation of porous carbons consisted of the impregnation of KOH followed by carbonization in N2. How the temperature and time for carbonization and the chemical ratio affected the properties of the resulting carbons was the major concern in the current study, whereas the influence of the impregnation temperature and time was not explored. The results of these investigations are discussed separately as follows. Effects of the Carbonization Temperature. One of the advantages presented by the chemical activation of coals compared to the physical activation is the lower temperature of the process. Previous studies6,10,15,19 have shown that chemical activation of bituminous coals with ZnCl2 or H3PO4 gives the highest porosity of the resulting activated carbon at a carbonization temperature around 500 °C, which is much lower than that for physical activation.20,21 The increase in temperature above 500 °C for chemical activation with ZnCl2 or H3PO4 may induce not only a weight loss but also a shrinkage in carbon structure, leading to a reduction, as well as a narrowing, in porosity. The structural contraction above 500 °C suggests that cross-links formed at low temperatures by treatment with ZnCl2 or H3PO4 do not own a high thermal stability.
Figure 1. Adsorption isotherms of N2 on porous carbons prepared from the carbonization of KOH-treated coal at different temperatures (prior to the carbonization, which lasted for 0 h at the carbonization temperature, the BW coal was impregnated with KOH to have a chemical ratio of 4.25).
The effect of the carbonization temperature on the porosity development of the carbons prepared from KOH activation was examined. The temperature range of carbonization employed in the present work was 5001000 °C. Figure 1 shows the typical isotherms of N2 adsorption on the carbons prepared from the BW coal at different carbonization temperatures with zero carbonization time. The chemical ratio of KOH to coal was fixed at 4.25. These isotherms show that the carbons are mainly microporous. The adsorption capacity increases with the carbonization temperature at the lowtemperature regime and reaches a maximum at 800 °C. Above 800 °C the N2 adsorption decreases with the increasing carbonization temperature. The decrease at high temperatures indicates the collapse of pore structures upon thermal treatment. Obviously, the crosslinks induced by KOH activation are more thermally resistant than those derived from ZnCl2 or H3PO4 treatment. Figure 2a shows the carbon yield as a function of the carbonization temperature. It can be seen that the carbon yield is a decreasing function of the carbonization temperature at low temperatures, reflecting the increased volatile evaluation with temperature. The decrease with temperature levels off when the temperature is raised to 700 °C. However, at temperatures above 800 °C, the carbon yield shows again a significant decrease with the increased temperature. The decrease in carbon yield at high temperatures can be attributed to the carbon gasification by CO2 or oxygen in the alkali,17 which will be discussed in following paragraph. The adsorption isotherms in Figure 1 are employed to deduce the surface characteristics of the carbons, and the results are shown in Figures 2 and 3. As shown in Figure 2, both surface area and pore volume are low for the carbon activated at 500 °C, and they increase rapidly with the increasing carbonization temperature to a maximum at 800 °C. The maximum values of the surface area and pore volume are more than 3000 m2/g and 1.5 cm3/g, respectively. The increase in porosity by raising the carbonization temperature from 500 to 700 °C can be attributed to the release of volatiles, which would result in pore formation.19,22 For carbonization temperatures above 700 °C, the release of CO2 from K2-
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2949
Figure 2. Effects of the carbonization temperatures on (a) carbon yield, (b) surface area, and (c) total pore volume of carbons prepared from KOH activation (prior to the carbonization, which lasted for 0 h at the carbonization temperature, the BW coal was impregnated with KOH to have a chemical ratio of 4.25).
CO3 formed during carbonization becomes significant.17 The evolved CO2 can react with carbon atoms to open up closed pores and enlarge existing micropores, resulting in the increase in porosity. Furthermore, it has been reported that at a temperature higher than 700 °C, the interactions between carbon and the potassium-containing compounds (such as K2O and K2CO3) become important,16,23 thus leading to carbon gasification and K-metal formation. These metallic potassiums may intercalate to the carbon matrix, resulting in the increase in pore volume. However, it has been reported that potassium metal intercalation compounds are not stable under gasification conditions.24,25 The metal can either sublimate or interact with some functional groups on the carbon surface to form catalytic active sites for gasification. At temperatures higher than 800 °C, the surface area and pore volume were found to decrease with the carbonization temperature. The decrease can be attributed to the breakdown of cross-links in the carbon matrix, with a consequent rearrangement of carbonaceous aggregates and the collapse or shrinkage of pores. The pore size distribution in Figure 3 shows that both micro- and mesopore volumes have a maximum value at a carbonization temperature of 800 °C. The trends of variation in both micro- and mesopore volumes are similar to that in the total pore volume shown in Figure 2, except that the increase of the carbonization temperature from 900 to 1000 °C shows a slight increase in
Figure 3. Effects of the carbonization temperatures on (a) micropore volume, (b) mesopore volume, and (c) average pore diameter of carbons prepared from KOH activation (prior to the carbonization, which lasted for 0 h at the carbonization temperature, the BW coal was impregnated with KOH to have a chemical ratio of 4.25). The uncertainty of the measurements was within (7%.
mesopore volume. This increase at high temperatures can be attributed to pore enlargement from gasification, since pore widening, rather than pore deepening, is favored at high temperatures.20 Because of the similar trends in the development of micro- and mesopore volumes, the carbonization temperature does not have a significant influence on the average pore diameter, as shown in Figure 3c. However, owing to the pore widening from gasification, the carbon prepared from 1000 °C carbonization possesses the largest average pore diameter. Effect of the Ratio of Chemical Reagent to Coal. Prior to carbonization, the impregnated samples with different chemical ratios (KOH/coal) were prepared by treating the coal with KOH solutions of different concentrations. It has been reported that the chemical ratio can significantly affect the pore development during activation.6,23,26 The effects of the ratio on the pore structures of the resulting carbons were explored, and the results are shown in Figures 4 and 5. In these figures, the final carbons were obtained by carrying out the carbonization at 700 and 800 °C with zero holding time. In the aspect of the chemical ratio dependence of carbon yield, Figure 4a shows that for carbonization performed at 700 °C the carbon yield increases with the increase of the chemical ratio from zero to unity. The
2950 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999
Figure 4. Effects of the chemical ratio of KOH to coal on (a) carbon yield, (b) surface area, and (c) total pore volume of carbons prepared by carbonization at 700 and 800 °C (the carbonization lasted for 0 h at the carbonization temperature).
Figure 5. Effects of the chemical ratio of KOH to coal on (a) micropore volume, (b) mesopore volume, and (c) average pore diameter of carbons prepared by carbonization at 700 and 800 °C (the carbonization lasted for 0 h at the carbonization temperature). The uncertainty of the measurements was within (7%.
zero chemical ratio represents that pure water was employed in the impregnation process. The increase in the carbon yield has been attributed to the dehydrogenation properties of KOH that decrease pyrolytic decomposition.17 As the ratios are higher than unity, the carbon yield was found to decrease with the increasing chemical ratio, indicating that carbon gasification by CO2 or potassium-containing compounds was enhanced by the increase in the KOH content. Figure 4 also shows that at various chemical ratios the carbon yields are similar for the carbonization performed at 700 and 800 °C. This is not surprising since the heating rate was high in these experiments, and thus the carbon gasification enhanced by raising the temperature from 700 to 800 °C was negligible. For the variation of surface characteristics with the chemical ratio, Figure 4 shows that for the samples carbonized at 700 °C the surface area and total pore volume are very low with an impregnation performed at zero chemical ratio. However, these values increase with the increasing chemical ratio and reach a maximum at a ratio of 6. The results are consistent with the findings of other workers6,17,23,26 who have suggested that increasing the amount of KOH leads to an increase in surface area and pore volume. The increase in the KOH content of the impregnated sample may promote the dehydrogenation of the coal in the subsequent carbonization and thus leads to the formation of a rigid matrix, less prone to volatile evolution and volume
contraction upon heating, resulting in a better porosity development.17 The increased amount of KOH impregnation may also enhance the formation of CO2 and potassium compounds, which would subsequently gasify the carbon to increase the porosity. Furthermore, the ash removal by caustic digestion may be enhanced by the increase in KOH.17 The reduced ash content would result in higher values of the measured surface area and pore volume since the ash has negligible specific surface area and pore volume compared to microporous carbons. To clarify how the chemical ratio affects the ash content of the resulting carbons, the amounts of ash retained in the carbons were evaluated and the results are shown in Figure 6. It can be seen from the figure that a significant amount of ash was removed by the treatment with KOH, and the amount of ash removal increases with the chemical ratio for the ratios less than 2. The amount of ash retained became unaffected by the chemical ratio as the ratio was further increased, indicating that the ability in caustic digestion of ash reaches its limit at a chemical ratio of 2. The ash content of each carbon can be calculated according to the carbon yield as well as the amount of ash retained. It was found that, on the basis of the data in Figures 4 and 6, the ash contents of the carbons derived from KOH activation are relatively low (
