Energy & Fuels 2001, 15, 1383-1386
1383
High-Surface-Area Activated Carbon from Chinese Coal Yong Zou* and Bu-Xing Han Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, P. R. China Received December 20, 2000. Revised Manuscript Received August 1, 2001
Activated carbons from Chinese coals have been prepared by chemical activation technology with potassium hydroxide. This paper analyzed the following aspects: raw material and its particle size, preoxidation treatment of the raw material, activation agent, activation temperature and time, and activation agent/coal ratio. Activated carbon with BET N2 surface area of the order of 2400 m2/g, substantal microporosity and total pore volume of over 1.40 cm3/g, and methylene blue adsorption capacity of over 440 mg/g, was obtained.
1. Introduction Activated carbon consumption is continuously increasing, because activated carbons are used in many processes, such as wastewater treatment, air purification, organic solvent recovery, gas separation and purification, heterogeneous catalysis, etc.1-4 Generally, there are two main sources for the production of commercial activated carbons: coal and lignocellulosic materials. A current trend in the preparation of activated carbons is the use of various kinds of natural coals, because they are cheap and readily available. Among a wide range of coals precursors, anthracites are much more of interest because of their low cost, high carbon content, and molecular structure.5 Basically, activated carbons can be produced by physical activation and chemical activation processes.1,2,4 Important advantages of the chemical activation technology with respect to physical activation routes are the lower activation temperature, shorter activation time, and higher char yield.1,2,4 Several chemicals have been reported to be useful as activation agents. The most widely used activation agents are phosphoric acid, zinc chloride, and alkali metal compounds.1,2,4-28 Phosphoric acid, zinc chloride are usually used for the activation of lignocellulosic materials which have not been previously carbonized.20-21 Contrarily, the alkali metal compounds, particularly potassium hydroxide, are used for the activation of coals and chars. Furthermore, potassium hydroxide has been found to be one of the most effective compounds to produce high-surface-area activated carbons.1,2,4,6-19,22-28 The objective of this study is to discuss the chemical activation of Chinese coals by potassium hydroxide. A series of variables were evaluated: coal rank, coal * Author to whom correspondence be addressed: E-mail: zou_yong@ yahoo.com. (1) Smisek, M.; Cerny, S. Active Carbon, Manufacture, Properties and Application; Elsevier: Amsterdam, 1970. (2) Bansal. R. C.; Donnet, J. B.; Stoeckli, H. F. Active Carbon; Dekker: New York, 1988. (3) Yang, R. T. Gas Separation by Adsorption; Butterworth: Stoneham, MA, 1987. (4) Porosity in Carbons: Characterization and Applications; Patrick, J. W., Ed.; Edward Arnold: London, 1995.
particle size, coal preoxidation, activation agent, activation agent/coal ratio, activation temperature, and time. 2. Experimental Section 2.1. Material. The Ning Xia anthracite (mined time: 20 years), Shen Mu coal (mined time: 10 years) (bituminous), and Lu An coal (mined time: 5 years) (lignite) were used as starting materials. Potassium hydroxide (92% purity, produced by Beijing Chemical Factory) was used as activation agent. The coal samples were ground and sieved to produce batches with particle sizes within 24-48, 48-180, and >180 mesh. The proximate analyses of the three coals studied are listed in Table 1 2.2. Production of Activated Carbon. The coals were used as feedstock and potassium hydroxide as an activation agent for the production of activated carbons. The production of activated carbons by a chemical activation technique was completed in a bench scale tubular activation reactor with a horizontal furnace (homemade). The processing steps used to produce the activated carbons were as follows: about 10 g of coal was mixed, while stirring, with an amount of a solution which contains amount of activation agent (KOH) depending on the ratio of activation agent/coal (A/C) desired. This mixture was dehydrated at 400 °C for 1.5 h and subsequently activated at 600-1000 °C under nitrogen. After activation, the sample was immediately submerged in deionized water, filtered, rinsed in deionized water, filtered again to remove any KOH and KOH derivatives, and finally dried at 110 °C for 4-8 h. Figure 1 presents a schematic of the process. 2.3. Characterization of the Activated Carbon. The color adsorption capacity is of activated carbon, and is tested in terms of methylene blue in P. R. of China. Generally, the higher the amount of methylene blue adsorbed, the better the quality of the activated carbon. The methylene blue test was conducted after the standard test of P. R. of China. The information about the standard test in detail as follows: 0.1 g (precision: 0.01 g) of dried pulverized (under 200 mesh) sample is added to a amount of methylene blue solution of 1.5 g/L in a 100 mL flask with agitating for 20 min, then the mixture is filtered. The dulling value of the filtered solution at 665 nm is measured by a photoabsorption method. When the value of the solution is equal to the value of cupric sulfate standard solution, the added amount of the methylene blue solution is methylene blue adsorption capacity (MBAC). Preparation of cupric sulfate standard solution is: 4 g (precision: 0.001 g) of CuSO4‚5H2O is solved in deionized water in a 1000 mL volumetric flask.
10.1021/ef0002851 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/19/2001
1384
Energy & Fuels, Vol. 15, No. 6, 2001
Zou and Han
Table 1. Composition of the Three Original Coals Studied
sample
moisture (%)
ash (%)
volatile matter (%)
fixed carbon (%)
Ning Xia anthracite Shen Mu coal Lu An coal
1.74 1.81 1.85
5.67 10.0 11.2
5.76 6.83 7.24
86.81 81.36 79.71
The characterization of activated carbons was also carried out through physical adsorption of nitrogen gas at 77 K, using a conventional volumetric system (Micromeritics ASAP 2400 instrument). The apparent surface area based on nitrogen isotherms was calculated using the BET equation, and using the “t” method accordingly to Harkins and Jura for the calculation of pore volume.29
3. Results and Discussion 3.1. Effect of Raw Materials. The activation conditions chosen for comparing to three Chinese coals studied (particle size 24-48 mesh) are: activation temperature, 810 °C; activation time, 120 min; activation agent, KOH; and weight ratio of activation agent/ coal (WKOH/Wcoal) R ) 3. Their yield (wt %) and methylene blue uptakes are listed in Table 2. It is shown that the activated carbon from Ning Xia anthracite has the highest methylene blue adsorption capacity and yield, because it has a high carbon content and a substantial fraction of pores with sizes closed to molecular dimensions compared to methylene blue. This result agreed with the reported by Illan-Gomez et al.16 In a previous study,30 the effect of particle size of the raw material on its activatation by KOH was studied. In the current study, the Ning Xia anthracite was selected in order to analyze the effect of the particle size during the activation process. Batches with three difference particle sizes of the Ning Xia anthracite were activated with the following conditions: KOH at 810 °C, 120 min, and activation agent/coal ratio R ) 5. Table 3 presents the results. It was shown that smaller sizes yield higher methylene blue adsorption capacities, and lower yields of activated charcoal, indicating that a preliminary grounding step improves the characteristics of the activated carbons. Preoxidation treatments can also improve the characteristics of the activated carbons produced from coal by physical or chemical activation technology.13,31-32 Several oxidation agents have been reported to be useful (5) Parra, J. B.; de Sousa, J. C.; Pis, J. J.; Pajares, J. A.; Bansal, R. C. Carbon 1995, 33, 801. (6) Wennerberg, A. N.; O’Grady, T. M. U.S. Patent 4,082,694, 1978. (7) Marsh, H.; Crawford, D. C. Carbon 1984, 22 (6), 603. (8) Ehrburger, P.; Addoun, A.; Addoun, F.; Donnet, J. B. Fuel 1986, 65, 1447. (9) Verheyen, V.; Jagtoyen, M.; Derbyshire, F. J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, 414. (10) O’Grady, T. M.; Wennerberg, A. N. ACS Symp. Ser. 1986, 303, 302. (11) Guy, J. P.; Verheyen, T. V.; Felber, M. P.; Heng, S.; Perry, G. J. Proceedings of the Australian Coal Science Conference, Brisbane, 1990; p 380. (12) Verheyen, V.; Rathbone, R.; Jagtoyen, M.; Derbyshire, F. Carbon 1995, 33, 763. (13) Audley, G. J.; Holder, K. A. EP Pat. 0216599, 1989. (14) Lebgaa, D.; Ehrburger, P.; Papirer, E.; Donnet, J. B. Bull. Soc. Chim. Fr. 1994, 131, 763. (15) Illan-Gomez, M. J.; Garcia-Garcia, A.; Salinas-Martinez de Lecea, C.; Linares-Solano, A. Energy Fuels 1996, 10, 1108. (16) Jagtoyen, M.; Thwaites, M.; Stencel, J.; McEnaney, B.; Derbyshire, F. Carbon 1992, 30, 1089.
Figure 1. Technological scheme for producing high-surfacearea activated carbons from Chinese coals. Table 2. Results of KOH Activation of the Three Chinese Coals Studied index yield (%) MBAC* (mg/g)
Ning Xia anthracite Shen Mu coal Lu An coal 65.62 306
52.96 225
51.90 120
Table 3. Results of KOH Activation of Three Particle Sizes of Ning Xia Anthracite index
24-48 mesh
48-180 mesh
>180 mesh
yield (%) MBAC* (mg/g)
56.56 443
52.63 454
51.37 460
for this purpose. The most widely used agents are nitric acid and air.13,31-32 In this study, nitric acid was selected as an oxidation agent. The oxidation process was carried out using different concentrations of nitric acid solution (from 0.5 to 2.0 N) at 25 °C for 24 h with a weight/ volume ratio (W/V) 1:4. After the oxidation process, the samples were washed with distilled water until they were free of nitrate ions. Once the nitrate ions were removed, the samples were dried at 110 °C for 10 h, and then they were activated with KOH at 810 °C for 120 min, and activation agent/coal ratio of R ) 5. Figure 2 shows the results. It was easy to observe, comparing the five samples with same activation conditions, the positive effect of the HNO3 preoxidation treatment on the degree of KOH activation, but the best treatment was carried out with 1.5 N HNO3 in this study. The four HNO3-treated samples yielded different methylene blue adsorption capacities, increasing in order as follows: unoxidized < 2 N HNO3 < 0.5 N HNO3 < 1 N HNO3 < 1.5 N HNO3. Our results agreed with those reported by Verheyen.13 3.2. Effect of Activation Agent. Table 4 lists the effects of four activation agents on the characteristics of the activated Ning Xia anthracite. In this table, four activated carbons were produced using the same activation conditions (activation temperature, 810 °C; activa-
Activated Carbon from Chinese Coal
Energy & Fuels, Vol. 15, No. 6, 2001 1385
Figure 2. Effect of the concentration of nitric acid solution on the methylene blue adsorption capacity and the yield of activated carbons prepared with KOH at 810 °C for 120 min and a ratio KOH/coal ) 5.
Figure 3. Relationship between methylene blue adsorption capacity, yield and the ratio of activation agent/coal used.
Table 4. Comparison of Four Activation Agents for the Preparation of Activated Ning Xia Anthracite index
KOH
K2CO3
NaOH
Na2CO3
yield (%) MBAC* (mg/g)
58.20 380
82.63 45
67.96 126
89.12 18
tion time, 120 min.; ratio of activation agent/coal, R ) 4). It was shown that activated carbons made using potassium hydroxide and sodium hydroxide had much higher methylene blue adsorption capacity and lower yield, but the activated carbon produced using potassium hydroxide gave the highest adsorption capacity. Our results agree with those reported in the literature.6,14,15,30,33 3.3. Effect of Activation Agent/Coal (A/C) Ratio. The ratio of agent/coal employed for activation widely varies in the literature.6-16,23,26-28 In the present work, ratios between 3/1 and 7/1 have been tested with the Ning Xia anthracite using potassium hydroxide as activating agent. The results for the Ning Xia anthracite activated with KOH at 810 °C for 120 min are given in Figure 3. It has been recognized that the greater the amount of potassium hydroxide used, the higher the methylene blue adsorption capacity and the lower the yield of activated carbon, but for the adsorption capacity, there was an optimum ratio of activation agent/coal (R ) ∼5). This result was consistent with those of Otowa et al.34 and Takahiro and Gunji.35 Figure 4 presents the N2 adsorption-desorption (17) Teng, H. S.; Hsu, L. Y. Carbon 1998, 36, 1387. (18) Ahmadpour, A.; Do, D. D. Carbon 1997, 35, 1723. (19) Caturla, F.; Molina-Sabio, M.; Rodriguez-Reinoso, F. Carbon 1991, 29, 999. (20) Rodriguez-Reinoso, F.; Molina-Sabio, M. Carbon 1992, 30, 1111. (21) Ahmadpour, A.; Do, D. D. Carbon 1996, 34, 471. (22) Evans, M. J. B.; Halliop, E.; MacDonald, J. A. F. Carbon 1999, 37, 269. (23) Ahmadpour, A.; Bradley, A. K.; Do, D. D. Ind. Eng. Chem. Res. 1998, 37, 1329. (24) Hsu L. Y.; Teng, H. S. Fuel Process. Technol. 2000, 64, 155. (25) Amarasekera, G.; Scarlett, M. J.; Mainwaring, D. E. Carbon 1998, 36, 1071. (26) Teng, H. S.; Hsu L. Y. Ind. Eng. Chem. Res. 1999, 38, 2947. (27) Jagtoyen, M.; Groppo, J.; Derbyshire, F. Fuel Process. Technol. 1993, 34, 85. (28) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982. (29) Wenming, Q.; Qinfeng, C. Tan Su Ji Shu (in Chinese) 1994, (2), 1.
Figure 4. Nitrogen adsorption-desorption isotherm (77 K) of activated carbons from the Ning Xia anthracite and effect of the ratio of activation agent/coal. (KOH activated at 810 °C for 120 min).
isotherms of two activated carbons prepared from Ning Xia anthracite using different ratios of activation agent/ coal (R ) 3 and 5) at the above activation conditions. It was easy to observe that (1) both almost have no hysteresis loop and exhibit “Type I ”, with their large uptakes indicating that they have a large degree of microporosity; (2) the shapes of the isotherms were quite different, as a result of different porosities in the two activated carbons. The extent of porosity (much higher for the sample activated at R ) 5), as well as the pore size distribution, vary considerably (the sample activated at R ) 5 presented a much wider N2 adsorption isotherm knee), thus providing evidence of the important effect of the ratio of activation agent/coal on the properties of the activated products. The structural characteristics of both activated carbons tested were summarized in Table 5. 3.4. Effect of Activation Temperature and Time. The advantages of chemical activation compared to (30) Samaras, P.; Diamadopoulos, E.; Sakellaropulos, G. P. Carbon 1994, 32, 771. (31) Serrano-Talavera, B.; Munoz-Guillena, M. J.; Linares-Solano, A.; Salinas-Martinez de Lecea, C. Energy Fuels 1997, 11, 785. (32) Otowa, T. DA 3932122 A (Germany), 1993. (33) Otowa, T.; Tanibata, R.; Itoh, M. Gas Separation Purification 1993, 7, 241. (34) Takahiro, K.; Gunji, M. U.S. Patent 5,143,889, 1992.
1386
Energy & Fuels, Vol. 15, No. 6, 2001
Zou and Han
Table 5. Typical Properties of Activated Carbons Obtained by KOH Activation of Ning Xia Anthracite activation agent/coal R 3 5 a
surface area SBET (m2/g)
micropore volume (cm3/ g) H-J
D-R
2194.7 0.9883 0.7651 2398.1 1.4084 0.8251
total pore volume MBAa ash yield cm3/g (H-J) mg/g wt % wt % 0.9949 1.4747
306 443
6.9 5.1
65.62 56.56
Methylene blue adsorption capacity.
physical activation are the lower activation temperature and the shorter activation time employed. In this work, the results for four activation temperatures (from 510 to 910 °C) and times (from 30 to 150 min) were compared. Figures 5 and 6 demonstrate the effects of activation temperature and activation time on the methylene blue adsorption capacity, and yield, respectively. It can be seen from Figure 5 that, in general, higher activation temperatures result in higher methylene blue adsorption capacities and in lower yields of activated carbon, although there is an optimum activation temperature (T ) ∼800 °C). This result was consistent with that of Otowa et al.34 Figure 6 shows that the longer the activation time, the higher the methylene blue adsorption capacity, and the lower the yield of activated charcoal, except during internal the time from 60 to 90 min., which is uncommon, and deserves some comments.
Figure 5. Relationship between methylene blue adsorption capacity, yield, and activation temperature employed.
4. Summary and Conclusions Three Chinese coals were proven to be feasible materials for activated carbon preparation by a chemical activation technology, and the Ning Xia anthracite was the best one among the materials tested, particularly for the preparation of high-surface-area activated carbon. Grinding and preoxidation of the coals favored the development of their porous structure and improved the characteristics of the activated carbons but not too much. Activated carbons with apparent surface areas over 2100 m2/g can be obtained using the Ning Xia anthracite. KOH is a very effective activating agent for the Ning Xia anthracite. In this study, it was also shown that the larger the amount of potassium hydroxide used, the
Figure 6. Relationship between methylene blue adsorption capacity, yield, and activation time employed.
higher adsorption capacity the activated charcoal has. The optimum ratio of activation agent/coal within the study was ∼5. High activation temperature and long activation time favored the development of the porous structure and improved adsorption capacity of the activated carbon. In this study, the optimum activation temperature was ∼800 °C. EF0002851
