Influence of Oxidation on the Preparation of Porous Carbons

Ind. Eng. Chem. Res. 2000, 39, 673-678

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Influence of Oxidation on the Preparation of Porous Carbons from Phenol-Formaldehyde Resins with KOH Activation Hsisheng Teng* and Sheng-Chi Wang Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan

The influence of oxidation on the production of high-porosity carbons from phenol-formaldehyde resins with KOH activation were examined under various preparation conditions. The activation process principally consisted of KOH impregnation followed by carbonization. Experimental results showed that prior to carbonization treating the resins with oxygen at 120 °C, either before or after KOH impregnation, enabled the enhancement of the yield of the carbon products. The porosity development was found to be hindered by conducting oxidation prior to the impregnation. For oxidation performed after the impregnation, at a low KOH/resin ratio the porosity was found to decrease upon oxidation, whereas the oxidation enhanced porosity development for activation performed at higher ratios. Varying the carbonization temperature and time did not show obvious influence on the effects of the oxidation. Introduction Porous carbons (or activated carbons) with high surface areas and pore volumes are produced from a variety of carbonaceous materials. They are among the most widely used adsorbents in the area of separation and purification of liquid or gas streams because of their high affinity and capacity displayed toward many compounds.1,2 In addition to serving as adsorbents, highporosity carbons have recently been used in the manufacture of high-performance double-layer capacitors.3-6 The applicability of these carbons mainly depends on their porous microstructure. The type of microstructure is determined by the choice of raw material and the conditions used for activation. Among various types of carbonaceous materials, coal is the most commonly used precursor for porous carbon production.7-9 Therefore, the preparation of porous carbons from coal has been extensively investigated. However, the considerably large amount of mineral species contained in coal generally affects the interactions of the activating agent with the carbonaceous components of coal as well as restricts the porosity development of the derived carbon. Phenol-formaldehyde resins have structural features similar to those in coals but contain much fewer mineral impurities. These impurities can be controlled to very low levels in the synthesis process.10,11 Thus, a thorough investigation and study on the porosity development of carbons prepared from phenol-formaldehyde resins can possibly shed some light on the process of preparation with coal. It is well-known that mixing and heating coal or lignocellulosic materials with an excess of alkali metal hydroxides produce a high-porosity carbon.12-14 These hydroxides were also found to be effective in creating the porosities of carbons derived from phenol-formaldehyde resins.15 In the case of bituminous coal, upon carbonization a certain portion of the coal will traverse through a plastic phase. The caking portion will present a much more ordered, less porous, and less reactive * To whom correspondence should be addressed. Tel.: 886-6-2385371. Fax: 886-6-2344496. E-mail: hteng@ mail.ncku.edu.tw.

structure than its noncaking counterpart.16 In the preparation of activated carbon from bituminous coal, it has been reported that introduction of oxygen into the coal structure can convert a caking coal from thermoplastic to thermosetting, thus promoting the porosity development.17-19 Oxygen functional groups appear to play a major role in promoting the crosslinking reactions between the building blocks of the coal. Although phenol-formaldehyde resins lack the caking behavior of bituminous coal, oxygen treatment to introduce oxygen functional groups will probably also affect the cross-linking reactions during carbonization of the resins. The application of oxygen treatment to improve the production of porous carbons with KOH activation was rarely reported in the literature. In the present study, phenol-formaldehyde resins were prepared and used as precursors of porous carbon. This report describes the effects of preliminary oxidation and how these effects are influenced by the variation of the activation conditions. Experimental Section Materials. Phenol-formaldehyde resins were used as the starting material for the preparation of porous carbons. The resins were synthesized under an environment of nitrogen at 95 °C, using an initial formaldehydeto-phenol ratio of 1.33 by mole. A base-catalyzed method was used in the synthesis, with ammonium hydroxide as the base. After synthesis, the resins were cured by heating in a vacuum for 2 h at 60 °C, followed by 12 h at 120 °C. The proximate and ultimate analyses of the resulting resins are shown in Table 1. Activation. Chemical activation of the resins was performed by using KOH as the chemical reagent. The activation process was initiated by mixing 1 g of the resins with a KOH solution containing 50 g of water. 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 resin (i.e., a chemical ratio) varying in the range 0-6 by weight. After mixing, the resin-KOH slurry was subjected to vacuum-drying at 110 °C for 24 h. The chemical-

10.1021/ie990473i CCC: $19.00 © 2000 American Chemical Society Published on Web 01/29/2000

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Table 1. Analysis of Phenol-Formaldehyde Resins ultimate (wt %, dry-ash-free basis) carbon nitrogen hydrogen oxygen

72.9 0.5 6.0 20.5

proximate (wt %, as-received) moisture volatile matter fixed carbon ash

1.0 58.6 40.4 0.0

loaded samples were then carbonized in a horizontal cylindrical furnace (60-mm i.d.) in a N2 atmosphere. Carbonization was carried out by heating the samples at 30 °C/min from room temperature to temperatures in a range of 500-900 °C, followed by holding the samples at the carbonization temperature for different lengths of 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. Leaching was carried out several times until the pH value of the water-carbon mixture was above 6. The leached products were then dried in a vacuum at 110 °C for 24 h, to give the final carbon products. Oxygen Treatment. Oxidation of the resins can be conducted either before or after the KOH impregnation. To examine the influence of the presence of KOH, both oxidation processes were tested. The oxidation was implemented in the cylindrical furnace by heating the samples in an oxygen atmosphere at a temperature ranging within 100-300 °C for 3 h. Carbonization of the oxygen-treated samples was performed in the same manner as that for the unoxidized samples. Carbon Characterization. Specific surface areas and pore volumes of the activated samples were determined by N2 adsorption at -196 °C. An automated adsorption apparatus (Micromeritics, ASAP 2010) was employed for these measurements. Nitrogen surface areas and micropore volumes of the samples were determined from the BET and Dubinin-Radushkevich (D-R) equations. 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.20 The average pore diameter can be determined according to the surface area and total pore volume, if the pores are assumed to be parallel and cylindrical. Results and Discussion Effects of Oxidation Conditions. In the present work various conditions for KOH activation were employed to prepare porous carbons. Preliminary experiments have shown that a preparation process involving impregnation with a KOH/resin ratio of 4 followed by carbonization at 700 °C for 2 h would produce highporosity carbons from the phenol-formaldehyde resins. This process was employed to examine the effects of oxidation conditions. Oxidation of the resins prior to KOH impregnation was performed at different temperatures, and the influence on the yield and surface characteristics of the carbon products was presented in Table 2. It can be seen that the carbon yield was significantly enhanced by oxygen treatment. The oxygen functional groups intro-

Table 2. Influence of Oxidation, Prior to Impregnation, on the Yield and Surface Characteristics of the Carbons from Phenol-Formaldehyde Resins with KOH Activationa avg. oxidation carbon pore pore size distrib. pore temp. yield BET SA volume micro meso diam. (°C) (%) (m2/g) (cm3/g) (%) (%) (nm) 120 200 no oxidation

36 31 22

1500 1100 2200

0.85 0.83 1.2

75 58 82

25 42 18

2.3 3.1 2.1

a The activation was performed by impregnating the resins with a KOH/resin ratio of 4, followed by carbonizing the sample at 700 °C for 2 h.

duced in the resin structure appear to play a role in promoting the cross-linking reactions between the polymers and thus enhance the carbon yield.21 The results also show that the yield was reduced by increasing the oxidation temperature from 120 to 200 °C, although the amount of oxygen introduced at 200 °C is expected to be higher. This indicates that increasing the oxygen content of the resins may result in enhanced carbon gasification during carbonization, thus reducing the carbon yield. As for the surface characteristics, Table 2 shows that the porosity development was found to be retarded by the introduction of oxygen. This can be attributed to the fact that oxidation promotes cross-linking reactions in the resins to construct a rigid matrix which restricts the accessibility of the resins to KOH during the impregnation process. The interaction of the resins with KOH was thus hindered, resulting in a reduced porosity development. Because of the restricted KOH distribution in the oxidized resins, the activation may mainly occur on the outer portion of the cross-linked structure, thus leading to a higher mesopore fraction as well as a larger pore diameter, as shown in Table 2. Oxygen treatment on the KOH-impregnated samples, prior to carbonization, was also conducted at various temperatures. Figure 1 shows the variations of the yield and surface characteristics of the activated products with the oxidation temperature. The data for the unoxidized sample are also reflected in the figure for the purpose of comparison. The carbon yield was found to increase upon oxidation and pass through a maximum at an oxidation temperature of 120 °C. The decrease in the yield with oxidation temperature in the hightemperature regime can be attributable to the increase in oxygen uptake, thus resulting in enhanced gasification during carbonization. It is of interest to notice from Table 2 and Figure 1 that both the oxidation processes, conducted either before or after the KOH impregnation, have similar effects in promoting the carbon yield. This indicates that the formation of the oxygen functional groups that are responsible for cross-linking to increase carbon yield is not affected by the presence of KOH. As for the surface characteristics, both the surface area and pore volume were found to increase through oxidizing the impregnated samples at temperatures lower than 200 °C. The outcome was opposite to that for oxidation performed prior to impregnation. This suggests that oxidizing the impregnated sample enables KOH to be embedded in the cross-linked structure induced by the oxidation, thus increasing the porosity. Similar to the results in carbon yield, a maximum increase in porosity development occurs at 120 °C for oxidation. Above this temperature, the porosity of the

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Figure 2. Effects of the carbonization time on the carbon yield and surface characteristics of carbons prepared with oxidation at 120 °C and with no oxidation (prior to the oxidation, which was followed by carbonization at 700 °C, the resins were impregnated with KOH to have a chemical ratio of 4).

Figure 1. Variations of the yield and surface characteristics of carbon products with the oxidation temperature (prior to the oxidation, the resins were impregnated at a KOH/resin ratio of 4; the oxidized samples were carbonized at 700 °C for 2 h).

carbon products decreases with the increasing oxidation temperature. This can be attributed to the increased loss of the reagent. It has been reported that, during impregnation, oxygen-containing groups, such as carboxyl and phenolic groups, can be neutralized by hydroxyl ions in the alkali, thus leading to the consumption of the reagent.14 In the process of carbonization the alkali reagent is involved not only in the dehydrogenation of the resins to form cross-linked structures15 but also in the oxidation and gasification reactions to increase the porosity of the carbons.22 Therefore, it is very likely that extensive oxidation at high temperatures to promote the oxygen population results in a significant consumption of KOH in neutralizing the oxygen functional groups, and thus leads to a reduction in porosity development. The pore size distribution was not significantly affected by this oxidation process and the results are not shown in this work. Because oxidation of the KOHloaded sample at 120 °C was found to be most effective in promoting the carbon yield and porosity development, this process was employed to further investigate how the oxidation effects are influenced by the variation of the activation conditions. Influence of Different Carbonization Conditions on the Oxidation Effects. The influence of different carbonization times and temperatures on the effects of oxidation was explored and the results are presented in Figures 2 and 3, respectively. Figure 2 shows that at

700 °C the carbon yield is a decreasing function of the carbonization time. The decreased yield can be attributed to the increased extent of carbon oxidation by KOH and its derivatives such as K2O and K2CO3.9,15,23 The oxides will be further gasified during carbonization or removed in the acid-washing step. The CO2 released from K2CO3 might also play a role in carbon gasification.12 In the aspect of carbonization temperature, because of volatile evolution and carbon gasification, the carbon yield was found to decrease with the temperature, as shown in Figure 3. The results in Figures 2 and 3 have shown that the carbon yield was significantly enhanced by the oxidation, and the variation in carbonization time and temperature had little influence on this effect. As for the porosity development, it can be seen from Figure 2 that at 700 °C both surface area and pore volume pass through a maximum at a carbonization time of 2 h, regardless of the conduction of oxidation. The increase in both surface area and pore volume with the increase in the carbonization time from 0 to 2 h can be mainly attributed to the increased carbon oxidation and gasification, as indicated by the decrease in carbon yield, that promotes porosity development. The decrease in porosity with the increased carbonization time from 2 to 3 h implies that extensive gasification of a carbon particle to diminish its porosity becomes appreciable.24 The figure also reflects that the micropore fraction decreases with the carbonization time, indicating the widening of micropores by gasification. It can be seen from Figure 3 that because of enhanced volatile release and carbon gasification,25 the porosity was found to increase with the carbonization temperature to a maximum at 700 °C for both oxidation and unoxidation cases. At temperatures above 700 °C, the porosity decreases with the increasing temperature for the oxidized samples. The decrease can be attributed to the severe thermal treatment causing the break down of the carbon matrix and extensive gasification at high temperatures, thus leading to the destruction of pore structures.26 For the unoxidized samples, the carbon was completely gasified during carbonization at 800 °C.

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Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000 Table 3. Influence of Oxidation, after Impregnation, on the Yield and Surface Characteristics of the Carbons from Phenol-Formaldehyde Resins with KOH Activation at Various Chemical Ratiosa (Data Shown in Brackets Are Obtained from Preparation with No Oxidation) carbon chemical yield ratio (%) 1 2 4 6

BET SA (m2/g)

pore volume (cm3/g)

pore size distrib. micro (%)

meso (%)

67 [54] 960 [1200] 0.50 [0.58] 91 [95] 9 [5] 60 [29] 1300 [1600] 0.65 [0.84] 87 [88] 13 [12] 35 [22] 2800 [2200] 1.5 [1.2] 81 [82] 19 [18] 16 [0] 2500 [NDb] 1.5 [ND] 70 [ND] 30 [ND]

avg. pore diam. (nm) 2.0 [2.0] 2.0 [2.1] 2.1 [2.1] 2.4 [ND]

a The carbonization was performed at 700 °C for 2 h. b Not detectable.

Figure 3. Effects of the carbonization temperature on the carbon yield and surface characteristics of carbons prepared with oxidation at 120 °C and with no oxidation (prior to the oxidation, which was followed by carbonization for 2 h, the resins were impregnated with KOH to have a chemical ratio of 4).

Figure 4. Effects of the carbonization temperature on the carbon yield and surface characteristics of carbons prepared with oxidation at 120 °C and with no oxidation (prior to the oxidation, which was followed by carbonization for 2 h, the resins were impregnated with KOH to have a chemical ratio of 2).

This indicates that the cross-linked structures induced from oxidation are more thermally stable than those in the unoxidized sample. Figure 3 also reflects that the proportion in the micropore volume generally decreases with the temperature, indicating the widening of micropores by gasification. The results in Figures 2 and 3 have demonstrated that oxidizing a KOH-loaded sample to promote the yield and porosity development of the final carbons is applicable for carbonization implemented under different conditions. However, the preceding experiments performed are restricted to samples impregnated at a KOH/resin ratio of 4. How the effects of oxidation vary with the chemical ratio will be investigated in the following section. Influence of Different KOH/Resin Ratios on the Oxidation Effects. To explore the influence of the chemical ratio, carbonization of samples impregnated

with a KOH/resin ratio of 2 was performed at various temperatures, and the results are shown in Figure 4. Similar to the activation with a chemical ratio of 4, Figure 4 shows that the carbon yield is a decreasing function of the carbonization temperature and its value is higher for the process with oxidation, indicating that oxidation to promote cross-linking reactions, and thus the carbon yield, is still effective at a lower chemical ratio. To have a thorough study on the influence of chemical ratio on the effects of oxidation, samples loaded with different amounts of KOH were prepared and then carbonized at 700 °C for 2 h for the purpose of comparison. The results are shown in Table 3, showing that the oxidation is able to enhance the yield for activation with different chemical ratios. Table 3 also shows that the carbon yield generally decreases with the increasing chemical ratio, indicating that carbon removal was enhanced by the increased amount of impregnation. As for the surface characteristics of the carbons prepared at a chemical ratio of 2, Figure 4 shows that, regardless of the conduction of oxidation, the surface area increases with the increasing carbonization temperature to a maximum at 800 °C and then decreases with a further increase of the temperature. The maximum surface area occurs at a temperature higher than that for the activation with a chemical ratio of 4, indicating that for samples impregnated with a smaller amount of KOH extensive gasification to destroy pore structures would occur at a higher temperature. The pore volume also passed through a maximum at 800 °C for the unoxidized sample, whereas for the oxidized sample the increase of temperature from 800 to 900 °C still results in an increase in pore volume, indicating that gasification causes the breaking through of pore walls27 and results in a decrease in surface area, but an increase in pore volume. Upon comparing with the results in Figure 3, it is of interest to observe from Figure 4 that both the surface area and pore volume are higher for activation without oxidation, except for carbonization performed at 900 °C. Obviously, in porosity development the effect of oxidation can be significantly affected by the chemical ratio. The influence of chemical ratio has been clearly revealed in Table 3. It can be seen that for a chemical ratio equal to or less than 2 the porosity was found to decrease upon oxidation, whereas oxidation results in the increase in porosity for activation with higher chemical ratios. The decrease in porosity with oxidation at lower chemical ratios can be attributed to the fact that a significant proportion of KOH impregnated was consumed in neutralizing the acidic oxygen functional groups introduced in the oxidation and the porosity development resulting from KOH activation was thus hindered.14

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However, at higher chemical ratios the amount of the chemical reagent consumed in digesting the oxygen functionalities generated in the oxidation is relatively low compared to the total amount loaded. Thus, the oxidation results in not only the promotion of carbon yield but also the increase in porosity development, as reflected in Figures 2 and 3. In Figure 4 the low porosity obtained from the unoxidized sample carbonized at 900 °C can be attributed to the extensive gasification of the carbon surface as well as the collapse of porous structures upon heating, to vanish the pores. Again, the data at high temperatures suggests that the cross-linked structures induced by oxidation are more thermally stable in the activation process. Similar to the results in Figures 2 and 3, the results in Figure 4 and Table 3 show that the oxidation has little influence on the pore size distribution for the samples activated with different chemical ratios. Because of the oxidation and gasification mechanisms during carbonization, the increase in carbonization temperature and chemical ratio generally causes the widening of pores and thus results in the decrease in micropore fraction, as shown in Figure 4 and Table 3. Conclusions This study has demonstrated that microporous carbons with high porosities can be prepared from phenolformaldehyde resins with KOH activation and conducting oxidation in the activation process results in a significant influence on the yield and surface characteristics of the resulting carbons. Oxidation of the resins, either before or after the KOH impregnation, is able to introduce oxygen functionalities that can cross-link the resin structures, thereby increasing the carbon yield. The effects of oxidation on surface characteristics are determined by the condition of the samples prepared. Conducting oxidation prior to KOH impregnation inhibits the development of porosity. This has been attributed to the fact that a cross-linked rigid matrix was induced by oxidation to restrict the penetration of KOH and thus the porosity development. For oxidation performed after KOH impregnation, the effects vary with the amount of KOH loaded. At a low KOH/resin ratio the porosity was found to decrease upon oxidation, whereas the oxidation enhanced the porosity development for activation performed at higher ratios. The decrease in porosity with oxidation at lower chemical ratios can be attributed to the fact that a significant portion of KOH impregnated is consumed in neutralizing the acidic oxygen functional groups introduced in the oxidation. At higher chemical ratios, the proportion of the loaded reagent consumed in digesting the oxygen functionalities is low, and as a result, the oxidation promotes not only the yield but also the porosity development. The results of the present work also reflect the fact that the variations in both the carbonization temperature and time have little influence on the effects of oxidation. Acknowledgment This research was supported by the National Science Council of Taiwan, through Project NSC 88-EPA-Z-006004.

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Received for review June 28, 1999 Revised manuscript received November 5, 1999 Accepted November 22, 1999 IE990473I