Remarkable Catalytic Activity of Calcined Pitch Based Activated

Pitch-based activated carbon fiber (pitch-based ACF) which is now commercially available has been .... H2O; (1) 0 vol %, (2) 5 vol %, (3) 7.5 vol %, (...
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Energy & Fuels 1997, 11, 272-276

Remarkable Catalytic Activity of Calcined Pitch Based Activated Carbon Fiber for Oxidative Removal of SO2 as Aqueous H2SO4 I. Mochida,* K. Kuroda, S. Miyamoto, C. Sotowa, Y. Korai, S. Kawano, K. Sakanishi, A. Yasutake,† and M. Yoshikawa‡ Institute of Advanced Material Study, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816, Japan Received September 18, 1996X

Catalytic activity of a series of pitch-based activated carbon fibers (ACFs) was examined at room temperature for the oxidative removal of SO2 as aqueous H2SO4 which was continuously recovered at steady state. Calcination up to 900 °C in an inert atmosphere increased the activity very markedly regardless of the ACF; however, higher temperatures up to 1200 °C increased the activity of ACFs with larger surface area (1000 m2/g) while the ACFs with smaller surface area lost activity very sharply. ACF with the largest surface area and calcined at 1100 °C resulted in the complete capture of 1000 ppm SO2 by ACF weigh/flow rate (W/F) 1 × 10-3 g‚min/mL under 10% H2O. Lower humidity reduced the activity, although larger W/F allowed complete removal of SO2. TPDE measurements revealed CO evolution from the ACF in the range of 400-1100 °C, the amount and the range depending on the ACF. ACF with the highest activity appeared to be obtained when the major CO-evolving groups were removed before marked reduction of the surface area took place. ACF that evolved CO more exhibited higher activity. The active site produced by CO evolution is discussed briefly.

Introduction Procedures for the efficient removal of SO2 in the urban atmosphere in places such as highway tunnel and parking lots with poor ventilation as well as in flue gases have been examined to protect the environment from acid rain and mist.1 The present authors have proposed the oxidative capture of SO2 room temperature as aqueous (aq) H2SO4 over activated carbon fiber (ACF).2,3 High activity for the complete capture of SO2 at low humidity is expected to recover H2SO4 in a more concentrated form and by using an ACF bed of smaller volume. Such SO2 removal by carbon materials is now studied extensively in Japan and USA.4-11 Since the * Author to whom correspondence should be addressed. † Chemical Research Laboratory, Technical Headquarters Nagasaki Research & Development Center, Mitsubishi Heavy Industries Ltd., 5-717-1 Fukahori-machi, Nagasaki 851-03,Japan. ‡ Research & Development Center, Osaka Gas Co., Ltd., 6-19-9 Torishima, Konohana-ku, Osaka 554, Japan. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Takeshita, M.; Soud, H. IEACR/58 FGD performance and experience on coal fired plants; IEA: London, 1993. (2) Kisamori, S.; Kawano, S.; Mochida, 1. Chem. Lett. 1993, 1899. (3) Kisamori, S.; Kuroda, K.; Kawano, S.; Mochida, I.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1994, 8, 1337. (4) Zang, W.; Yahiro, H.; Mizuno, N.; Izumi, J.; Iwamoto, M. Langmuir 1993, 9 2337. (5) Smith, D.-M.; Keifer, J.-R.; Noovicky, M.; Chughtai, A.-R. Appl. Spectrosc. 1989, 43,103. (6) Abramowith, H.; et al. Carbon 1976, 14, 84. (7) Davin, P. Carbon 1990, 28, 565. (8) Bruggendick, H.; Gilgen, R. 211th ACS National Meeting, Prepr. 1996, 41-1, 349. (9) Tsuji, K.; Shiraishi, 1. 211th ACS National Meeting, Prepr. 1996, 41-1, 404. (10) Jungten, H.; Richter, E.; Knoblauch, K.; Hoang-Phu, T. Chem. Eng. Sci. 1988, 43, 419. (11) Haure, P.-M.; Hudgins, R. R.; Silveston, P.-L. Chem. Eng. Sci. 1990, 43, 121.

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present procedure can be applicable for the removal of SO2 in the flue gas from the power plant, the operation temperature around 100 °C is favorable; however, continuous recovery of aq H2SO4 is rather difficult. Hence, the cost of heat exchange and efficiency of H2SO4 recovery must be balanced. At the present stage, the authors are searching for ACF of larger activity. Pitchbased activated carbon fiber (pitch-based ACF) which is now commercially available has been reported to have a very high surface area (around 2000 m2/g) and high SO2 adsorption capacity.12 However, the pitch-based ACFs in the as-received form show much lower activity for this reaction than poly(acrylonitrile) (PAN)-based ACF.2,3 The PAN-based ACF was found to have much higher activity after calcination at 800 °C.13 In the present study, a series of pitch-based ACFs calcined up to 1200 °C were examined for the oxidative removal of SO2 at 30 °C. These ACFs in the as-received form carried more oxygen functional groups which have higher thermal stability than those on PAN-based ACFs.14 Hence the calcination at higher temperature was expected to give higher activity. The calcination may improve the hydrophobicity of the ACF surface because the pitch is much more graphitic than PAN and reduces the amount of H2O required for the elution of aq H2SO4: thus, very high activity was expected to be obtained with pitch-based ACF. The calcination removes most of the oxygen functional groups from the surface and enhances the graphitization. Basic oxygen (12) Wang, Z.-M.; Kaneko, K. J. Phys. Chem. 1995, 99, 16714. (13) Mochida, I.; Hirayama, T.; Kisamori, S.; Kawano, S.; Fujitsu, H. Langmuir 1992, 8, 2290. (14) Mochida, I.; Kuroda, K.; Kawabuchi, Y.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Chem. Lett. 1996, 541.

© 1997 American Chemical Society

Catalytic Activity of Calcined Pitch Based ACF

Energy & Fuels, Vol. 11, No. 2, 1997 273

Table 1. Properties of Pitch-Based Active Carbon FIbers ACFs OG-5A OG-7A OG-8A OG-10A OG-15A OG-15A-H1000a OG-15A-H1100a OG-15A-H1200a OG-20A OG-20A-H800a OG-20A-H900a OG-20A-H1000a OG-20A-H1100a OG-20A-H1200a

ultimate analysis (wt %) C H N O ash 89.6 90.8 91.2 91.6 93.9 95.8 96.6 97.6 93.9 94.2 95.0 95.8 97.5 98.0

1.1 1.0 0.9 0.9 0.9 0.6 0.2 0.1 0.9 0.9 0.8 0.6 0.1 0

0.7 0.6 0.6 0.5 0.7 0.6 0.5 0.4 0.3 0.3 0.3 0.3 0.2 0.2

8.2 7.4 7.0 6.7 4.1 2.5 2.2 1.4 4.6 4.1 3.4 2.8 1.6 1.2

0.3 0.2 0.3 0.3 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.6 0.6

surface area (m2/g) 480 690 840 1060 1927 1547 1485 1307 2149 1997 1780 1721 1720 1683

a Calcination temperature (°C). OG-series: Pitch-based activated carbon fiber.

Figure 1. Reaction apparatus.

functional groups15 or the graphene edge of unsaturated valence may be produced by the evolution of such groups, to be active sites for the oxidation of SO2.16 Experimental Section Pitch-Based ACFs. A series of pitch-based ACFs, labeled OG, were supplied by Osaka Gas Co., Ltd. The properties are summarized in Table 1. The ACFs were calcined in N2 flow at 600-1200 °C for 1 h before use. SO2 Removal. SO2 removal was carried out at 30 °C using a fixed bed flow through a reactor which is illustrated in Figure 1. A sample of ACF was packed densely in the reactor tube (8 mm diameter). The length of ACF bed (0.25 g) was ca. 30 mm. The flow rate was fixed at 100 mL/min. A model flue gas containing SO2 at 1000 ppm, O2 5 vol %, and H2O 10-30 vol % in nitrogen was used. The H2SO4 was trapped at the outlet of the reactor as illustrated in Figure 1. A part of the H2O was found to condense as dew drops in the reactor when the inlet water vapor concentration was 10 vol %. Therefore, the concentrations of SO2 at the inlet and outlet gases were observed continuously by a frame photometric detector (R268Y; Hamamatsu Photonics). Temperature-Programmed Decomposition (TPDE). TPDE measurements of the as-received ACFs OG-5A, -15A, and -20A, were carried out using a quartz-tube apparatus equipped with a quadropole mass spectrometer (AQA-200; Nichiden-Anelva Inc.). Each sample (0.1 g) was heated in a flow of helium flow up to 1000 °C at a rate of 10 °C/min. The evolved gases such as H2, CO, and CO2 were continuously analyzed by the mass spectrometer. (15) Voll, M.; Bohem, H. P. Carbon 1970, 8, 741. (16) Mochida, I.; Kawano, S.; Hironaka, M.; Kawabuchi, Y.; Korai, Y.; Matsumura, Y.; Yoshikawa, M. Langmuir, in press.

Figure 2. Breakthrough profiles of SO2 over pitch ACFs. SO2 1000 ppm, O2 5 vol %, H2O 10 vol %. W/F ) 2.5 × 10-3 g‚min‚mL-1, reaction temperature 30 °C. (1) OG-5A (50 h); (2) OG-7A (50 h); (3) OG-8A (50 h); (4) OG-10A (50 h); (5) OG15A (50 h); (6) OG-20A (50 h).

Figure 3. Breakthrough profiles of SO2 over pitch ACFs. SO2 1000 ppm, O2 5 vol %, H2O 10 vol %. W/F: 2.5 × 10-3 g‚min‚mL-1, reaction temperature 30 °C. (1) OG-15A; (2) OG15A-H600; (3) OG-15A-H800; (4) OG-15A-H900; (5) OG-15AH1000; (6) OG-15A-H1100; (7) OG-15A-H1200.

Results 1. Activity of As-Received Pitch-Based ACFs. Figure 2 illustrates the breakthrough profiles of SO2 (at 1000 ppm) over as-received pitch-based ACFs at 30 °C. The H2O concentration and W/F were 10% and 2.5 × 10-3 g‚min/mL (weight of ACF was 0.25 g), respectively. In all cases, effluent SO2 was detected in less than 30 min, regardless of the types of ACF, 65 to 80% of SO2 being found at the outlet. Up to 35% of inlet SO2 was captured at steady state over as-received ACFs for at least 15 h continuously (recovered as aq H2SO4). Since the solubility of SO2 in water at 30 °C is 27.2 mL SO2(v)/ mL H2O(1) (30 °C), its contribution for SO2 removal is minor. Although the steady-state capture appears to be higher over ACFs of higher surface area, the differences were very small. The activities of these pitchbased ACFs were much lower than those of as-received PAN-based ACFs previously reported.3 2. Influence of Calcination on the Activity of Pitch-Based ACF. Figure 3 illustrates the SO2 breakthrough profiles under the same conditions as those of Figure 2 over OG-15A after calcining at a series of temperatures up to 1200 °C. The activity of the ACF increased with increasing calcination temperature up to 1200 °C. Calcination above 1000 °C was most effective and resulted in complete SO2 removal for at least 15 h. The complete removal was found to continue for 50 h in a separate test. Figure 4 illustrates the steady-state removal of SO2 under the same conditions to those of Figure 2 over a

274 Energy & Fuels, Vol. 11, No. 2, 1997

Figure 4. Effects of heat-treatment temperature for SO2 removal over pitch ACFs. SO2 1000 ppm, O2 5 vol %, H2O 10 vol %. W/F ) 2.5 × 10-3 g‚min‚mL-1, reaction temperature 30 °C. (O) OG-5A (SA 480 m2/g); (4) OG-7A (SA 690 m2/g); (]) OG-8A (SA 840 m2/g); (0) OG-10A (SA 1060 m2/g); (3) OG15A (SA 1930 m2/g); (x) OG-20A (SA 2150 m2/g).

Figure 5. Breakthrough profiles of SO2 over pitch ACFs. SO2 1000 ppm, O2 5 vol %, H2O 10 vol %. W/F: 1.0 × 10-3 g‚min‚mL-1, reaction temperature 30 °C. (1) OG-15A; (2) OG15A-H600; (3) OG-15A-H800; (4) OG-15A-H900; (5) OG-15AH1000; (6) OG-15A-H1100; (7) OG-15A-H1200.

series of ACFs calcined at 600-1200 °C. Calcination up to 900 °C increased markedly the activity of all ACFs. Temperatures higher than 900 °C decreased the activity for those ACFs with rather lower surface area than 1000 m2/g. In contrast, ACFs with surface area above 1000 m2/g further increased activity by calcination at still higher temperatures. The activity of the ACFs follows the same order as their surface areas. ACF-OG-20A with the largest surface area and calcined above 900 °C exhibited the highest activity and completely remove SO2. 3. Activity of ACFs of Large Surface Area. Figures 5 and 6 illustrate the breakthrough profiles of SO2 over OG-15A and OG-20A calcined up to 1200 °C. the reaction was conducted at a shorter contact time of 1.0 × 10-3 g‚min/mL. The effect of different calcination temperatures is more apparent which allows activity differences of such ACFs to be more readily distinguishted. Calcination at 1100 °C (H-1100) resulted in complete removal of SO2 while lower or higher temperatures allowed breakthrough of a considerable concentrations of SO2. The activity of OG-20A with the highest surface area exhibited a similar effects due to calcination, although its activity appeared higher than OG-15A for a given calcination temperatures. At a W/F of as small as 5 × 10-4 g‚min/mL, a steady state removal of SO2 of 95% was achieved over OG-20A-H1100 as shown in Figure 6.

Mochida et al.

Figure 6. Breakthrough profiles of SO2 over pitch ACFs. SO2 1000 ppm, O2 5 vol %, H2O 10 vol %. W/F: 1.0 × 10-3 g‚min‚mL-1, reaction temperature 30 °C. (1) OG-20A; (2) OG20A-H900; (3) OG-20A-H1000; (4) OG-20A-H1100; (5) OG-20AH1200; (6) OG-20A-H1100 (W/F 5.0 × 10-4 g‚min‚mL-1).

Figure 7. Breakthrough profiles of SO2 over heat-treated pitch ACF at several H2O concentrations. SO2 1000 ppm, O2 5 vol %. W/F: 1.0 × 10-3 g‚min‚mL-1, reaction temperature 30 °C. ACF: OG-20A-H1100. H2O; (1) 0 vol %, (2) 5 vol %, (3) 7.5 vol %, (4) 10 vol %.

Figure 8. Breakthrough profiles of SO2 over pitch ACF. SO2 1000 ppm, O2 5 vol %, H2O 5 vol %. Reaction temperature 30 °C. W/F: (1) 1.0 × 10-3 g‚min‚mL-1 OG-20A-H1100. (2) 2.5 × 10-3 g‚min‚mL-1 OG-20A-H1100. (3) 5.0 × 10-3 g‚min‚mL-1 OG-20A-H1100 (60 h). (4) 5.0 × 10-3 g‚min‚mL-1 OG-15AH1100.

4. Influence of H2O Concentration. Figure 7 shows the influence of H2O concentration on SO2 removal over OG-20A-H1100 at W/F of 1.0 × 10-3 g‚min/ mL. SO2 in dry air broke through completely within 1 h. Moisture in the career gas prolonged the period before the breakthrough and decreased the steady state concentration of outlet SO2; 5 and 7.5% H2O reduced the steady-state outlet concentrations to 40 and 2%, respectively, while 10% H2O was enough for complete removal of SO2 at the steady state. Figure 8 illustrates breakthrough profiles over OG20A-H1100 at 5% H2O, varying W/F from 1.0 × 10-3 to

Catalytic Activity of Calcined Pitch Based ACF

Energy & Fuels, Vol. 11, No. 2, 1997 275

Figure 10. Rates of SO2 removal vs amounts of evolved CO over OG-20A. Sample: as-received OG-20A. CO evolution: by 1100 °C in TPDE. SO2 removal: reaction temperature 30 °C, weight ACF 100 mg, SO2 1000 ppm, O2 5 vol %, H2O 10 vol %. Figure 9. TPDE spectra of CO and CO2 evolution from pitchbased ACF: weight 100 mg; flow rate 100 mL/min. Sample: thin-line, OG-5A; medium thick line, OG-15A; heavy line, OG20A. Table 2. Maximum Activity of SO2 Removal, Peak Temperature, and Amount of CO Evolution from ACFs ACFs

rate of SO2 removal (g SO2/min‚g ACF)

temp of max CO evoln (amt of evolved CO)

OG-5A OG-7A OG-10A OG-15A OG-20A

3.1 × 10-6 3.9 × 10-6 4.9 × 10-6 7.0 × 10-6 2.2 × 10-5

890 °C (599 mg) 870 °C (993 mg) 900 °C (937 mg) 910 °C (1405 mg) 880 °C (2286 mg)

5.0 × 10-3 g‚min/mL. Large W/F increased steady-state removal of SO2 at the fixed H2O concentration of 5%, W/F of 5.0 × 10-3 g‚min/mL allowing 100% removal over OG-20A-H1100. 5. TPDE Profiles of Pitch-ACFs. Figure 9 illustrates the TPDE profiles of CO and CO2 evolution from the three as-received pitch-based ACFs. The profiles of all pitch-based ACFs were similar in terms of the evolution ranges. CO2 started to evolve at 150 °C, reached a maximum at 250 °C, and then decreased to zero by 900 °C. CO started to evolve at 300 °C, reached a maximum at 900 °C, and then decreased to zero by 1100 °C. In contact, the temperature of H2 evolution was different with ACFs. The amount of evolved CO was certainly dependent upon the types of ACF. ACFs with larger surface area appear to evolve more CO and to extend CO evolution to higher temperatures. The amount of CO evolved from ACFs is summarized in Table 2. Figure 9 also illustrates the evolution H2 for ACFs, which started arround 850 °C. The temperature of the maximum evolution appeared to be higher with the ACF of the larger surface area. OG-5A of low surface area gave the maximum at 1000 °C, while ACFs of larger surface area (OG-15A and -20A) showed the maximum beyond 1100 °C, indicating different progress of the graphitization. Discussion The present study shows that a pitch-based ACF with the large surface area and calcined at 1100 °C exhibits remarkably high activity for the continuous removal of SO2 as aq H2SO4 at room temperature. This particular ACF can remove SO2 completely at W/F of 1 × 10-3

g‚min/mL under 10% absolute humidity. Less humidity (5%) required a large W/F of 5 × 10-3 g‚min/mL for complete removal. Such activity is much higher than that of PAN-ACF reported in previous papers.2,3 Pitchbased ACFs with smaller surface area exhibited lower activity. Nevertheless, calcination up to 900 °C increased their activity markedly. The cost of ACFs is almost proportional to their surface area because the surface area is proportional to the amount of burn off. The volume of the reactor, amount of H2O required for completely removal, and concentration of aq H2SO4 recovered are all influenced by the ACF’s activity. These factors also affect the price of the recovered product and ultimately the cost for performance. Hence it is necessary to balance cost and performance for optimization of ACFs. According to kinetic analyses of the SO2 removal over the ACF such as reaction order in SO2, H2O, and O2 in previous papers,17-19 the number of active sites for H2SO4 trapping over ACF and the rate constant for aq H2SO4 desorption determine the activity of ACF, where the rate of the desorption is a function of the number of the active sites and concentration of H2O. The active sites are not yet identified; however, the fact that heat treatment which evolves CO enhances an ACF’s activity suggests that the number of active sites may be proportional to the amount of CO evolved. Two possible correlations have been examined as follows: (1) the amount of CO evolution and the enhancement of activity by the heat treatment of OG-20A at various temperatures (Figure 10); (2) the amount of CO evolution and the rate of removal for ACFs at their optimum heattreatment temperature (Figure 11). Figure 10 indicates that the removal of major COevolving groups is related in enhancing the catalytic activity. The maximum activity of ACFs is proportional to the amount of CO evolution, indicating again that removing CO-evolving groups induces the formation of active sites. The evolution of CO2 was completed at lower temperature than that for the induction of the highest (17) Mochida, I; Kuroda, K; Kawabuchi, Y; Kawano, S.; Yasutake, A.; Yoshikawa, M.; Matsumura, Y. 211th ACS National Meeting, Prepr. 1996, 41-1, 335. (18) Mochida, I.; Kuroda, K.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Fuel, submitted for publication. (19) Mochida, I.; Kuroda, K.; Kawano; Kawano, S.; Yoshikawa, M.; Matsumura, Y.; Rodney, A.; Grulke, E. Fuel, submitted for publication.

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Figure 11. Rates of removal vs amounts of evolved CO from ACFs. ACFs: OG-5A, -7A, -10A, -15A, -20A calcined at their optimum temperature. Reaction conditions and CO evolution: see Figures 2 and 9.

Figure 12. Schematic diagram representing three types of carbonyl groups on the edges.

activity, ruling out its correlation to the active site. The evolution of H2 appears to intiate the decrease of the activity, being related to the progress of the graphitization. The rate of SO2 removal may reflect also the surface properties of the ACF. Hence the first correlation may be influenced by both the number of active sites and the desorption rate of aq H2SO4 over the ACFs of different oxygen contents. In the second case (Figure 10), the ACFs are almost completely oxygen free because the highest activities occur after the CO evolution is almost completed. Therefore, the number of active sites on these ACFs may be the key factor, although the

Mochida et al.

graphitization extent of calcined ACF may also vary according to the extent of activation and subsequent calcination conditions. Finally, an image of the active site for the present type of SO2 removal over the pitch-based ACF may be of value for discussion. Basic oxygen functional groups are assumed to be present on the surface after the heat treatment of ACFs.15 Such sites can adsorb SO2. However, the present study clarified that the surface of ACF with almost no oxygen functional groups is the most active. Hence the oxygen functional groups are not considered to be the active sites. The activated carbon is supposed to carry a number of carbonyl groups.20 Such carbonyl groups may be formed on three types of edges in the graphene plane as illustrated in Figure 12. The decomposition of type A (aromatic carbonyl) groups breaks the hexagonal sheet, while decomposition of types B and C (benzyl carbonyl) groups produces free valences, which may be localized by forming a benzyne type bond in case B or delocalized by conjugation in case C. Thus, case B may provide active sites for oxidation, although there is no direct evidence in our study. The rate for the desorption of aq H2SO4 depends on both the number of active sites as the trace of the oxygen functional groups and the extent of graphitization. The hydrophilic properties of the graphite may enhance the elution of aq H2SO4 from the ACF surface with less amount of H2O. Heat treatment upon the optimal temperature is favorable for both factors by removing oxygen function group groups and graphitization the ACF. However, too high a temperature of the heat treatment graphitizes the ACF, reducing the surface area and removing the active site. The different temperature range of H2 evolution may indicate that the extensive activation retards the progress of the graphitization to the higher temperature. The number of active sites and the rate constant for H2SO4 desorption will be analyzed kinetically in a future study. EF960160P (20) Donnet, J. B.; Bansal, R. C.; Wang, M. J. Carbon Black; Marcel Dekker, Inc.: New York, 1993; p 175.