Activated Carbon Fibers for Enviromental Applications

Veronica Carneiro, Claflin University Lisa Huggins, Claflin University Curtis Lee, Wake Forest University Joel Harris, South Carolina Governor's School for Science and Mathematics Academic Advisors: Dan Edie, Young-Seak Lee, Yulia Basova, and Seung Kon Ryu Industry Advisors: Ernie Romine and Mark Southard (Conoco)

Introduction Pollution continues to be a major threat to the ecology. Because of this, low-cost, lightweight, adsorbent materials are needed to remove large molecule pollutants such as proteins, radioactive material and other industrial emissions such as nitrogen and sulfur oxides. These latter gases are particularly damaging to the environment because they combine with atmospheric moisture and fall back to the earth as an acid precipitate. Powdered activated carbon is currently being used to remove contaminants in wastewater streams. However, powdered carbon cannot be easily processed into sheets or felt [1], the preferred form for low pressure-drop filters. This limits its use in high volume applications. Activated carbon fibers, on the other hand, can be readily formed into cloth, sheets, and felt. Thus, if properly formed, this new variety of fibers could be an ideal replacement for powdered activated carbon. According to Merraoui, pitch-based activated carbon fibers (ACFs) have a high carbon content and preferentially adsorb nonpolar organic vapors [2]. The large surface area and pore volume of ACFs give them excellent adsorption properties. The degree of adsorption depends on the pore size and distribution. Previous research has shown that adding small amounts of metals to the isotropic pitch precursor yields ACFs with relatively uniform mesopores (pores having larger diameters). These mesopore-containing ACFs could be attractive for removing biological environmental pollutants and for catalytic and electronic applications [3]. The goal of this project is to develop uniform mesoporous pitch-based ACFs for adsorbing environmental pollutants. The size of the mesopores formed on the surface of the ACFs must be complementary to the size of the pollutants adsorbed. Therefore, a series of experiments were conducted to determine the effect of precursor composition and process conditions on pore size and uniformity of ACFs. In these tests samples of pitch supplied by Conoco, Inc. and Korean colleagues at Chungnam National University were melt-spun into multi-filament tows. Based on previous research and these ongoing experiments, optimum conditions for producing the mesoporous ACF were determined. Fibers were spun from both pitch samples and from samples that contained small amounts of silver nitrate to evaluate the effect of metals on pore uniformity and size. Also, fibers with circular cross sections and trilobal cross sections were spun to determine the effect of cross-sectional shape on the total surface area of the ACFs. A Micromeritics® analyzer (BET) was used to measure the total-surface area, pore size, pore distribution and adsorption performance of the ACFs. The surface texture and porosity was monitored using a scanning electron microscope (SEM). The results should indicate the best precursor, fiber shape, and processes conditions for creating ACFs that contain a given pore size.

Experimental Procedures During this project experiments were conducted at Clemson University in Clemson, South Carolina and at Chungnam National University in Taejon, Korea. 1. Experiments Conducted at Clemson University, Clemson, South Carolina, U.S.A Mixing Using a mortar and pestle, approximately 200g of Conoco pitch and 20g of silver nitrate were ground to a fine powder. The pitch was divided into ten samples each weighing 20g. To each sample, 0.20g of silver nitrate was added. This yielded pitch samples containing a 1% silver nitrate concentration. The process was repeated using the Korea isotropic pitch to produce a second set of samples containing 1% silver nitrate. The Conoco and Korea samples containing 1% silver nitrate were thoroughly blended using a Rheomix mechanical mixer. The temperatures of each zone were set to approximately 10°C below the softening point of each pitch. The Conoco samples were melt-blended at 220°C and the Korea samples were melt-blended at 240°C. Rheology A mortar and pestle were used to grind the raw (no silver nitrate) and the mixed (1% silver nitrate) isotropic pitch samples into powder. These powder samples were formed into pellets in a Carver Press at a pressure of 5000 psi. These pellets measured approximately 4 mm in thickness and 15 mm in diameter. The pellets were placed in a Rheometric Scientific Rotational Rheometer (model RDS II) and steady state tests were conducted to determine the samples' viscosities at low rates of shear. Step rate default tests were conducted on the mixed Korea isotropic pitch sample to determine how the sample behaved with respect to torque at a constant shear rate for a given length of time. All samples were presheared to ensure an even distribution of the sample between the plate and the cone of the RDS II. Steady rate sweep tests were then conducted on all of the isotropic pitch samples and the results analyzed. From these results, graphs of viscosity versus shear rate were plotted. Melt Spinning The various Korea and Conoco isotropic pitch samples were formed into fibers using a bench-scale spinning apparatus and a nitrogen purge. The bench-scale apparatus consists of a cylinder with a piston on the top and the spinneret attached on the bottom. A pitch sample of about 50-80 grams was first loaded into the cylinder. The piston was then inserted in the top of the cylinder and the cylinder heated to the desired spinning temperature. Once the temperature of the pitch reached steady state, the piston was lowered at a preset speed, forcing the molten pitch through the spinneret. The cylinder was continuously purged with nitrogen to prevent the pitch from oxidizing at the spinning temperature. Mixed (silver nitrate) and unmixed round and trilobal fibers were spun. The silver nitrate-containing samples were spun 10-15°C higher than the unmixed samples. The trilobal fibers required spinning at temperatures that were 5-15°C lower than those used to spin the round fibers. Also, the spinning pressure was lower when spinnerets with higher cross-sectional areas were used (trilobal versus round capillaries). Stabilization Stabilization cross-links the as-spun pitch fibers, rendering them infusible during carbonization. Therefore, the fibers will not melt at higher heat treatments. This is accomplished by heating the fiber in a flow of oxygen so the molecules within the fibers become linked by the addition of oxygen. As a result, the fibers gained about 8% in weight during stabilization. Stabilization of the as-spun fibers was performed using a Fisher Scientific convection oven. The periods of time required for stabilization of the round and trilobal fibers are given in Table 1 and Table 2, respectively.

Table 1.Stabilization and carbonization conditions for round fibers. Round Type Conoco Pitch Conoco Mixed Pitch Korea Pitch Korea Mixed Pitch Softening Point [ºC] 230 255 Spinning Temperature. [ºC] 1) 940[ft/min] 245±1 263±2 271±1 290±1 2)1500[ft/min] 3)2100[ft/min] OxidationTemperature 225ºC /30hrs/ 0.5ºC /min 270ºC /2hrs/5ºC /min 300ºC /3hrs/5ºC /min 265ºC/13hr / 0.5ºC /min Carbonization Temperature 1000ºC / 1hr, 10ºC /min Table 2.Stabilization and carbonization conditions for trilobal fibers. Trilobal Type Conoco Pitch Conoco Mixed Pitch Korea Pitch Korea Mixed Pitch Softening Point [ºC] 230 255 Spinning Temperature. [ºC] 1) 940[ft/min] 245±1 263±2 265-266 264-265 2)1500[ft/min] 3)2100[ft/min] OxidationTemperature 225ºC /30hrs/ 0.5ºC /min 270ºC /2hrs/5ºC /min 300ºC /3hrs/5ºC /min 265ºC/13hr / 0.5ºC /min Carbonization Temperature 1000ºC / 1hr, 10ºC /min

Carbonization Carbonization of the fibers was conducted using an ASTRO® carbonization furnace. The stabilized fibers were carbonized by heating them to 1000oC in an inert gas (helium) atmosphere. The carbonization conditions for the round and trilobal fibers are given in Table 1 and Table 2, respectively. 2. Experiments Conducted at Chungnam National University, Taejon, Korea Mixing An additional precursor sample was prepared at Chungnam National University in Taejon, Korea. This consisted of mixing 100 g of Korea isotropic pitch with 3 g ofsilver nitrate using an electrical mixer, producing a 3 % metal containing isotropic pitch sample. Melt Spinning This additional sample was melt spun at Chungnam National University using a custom- made spinning apparatus, yielding fibers containing 3 % silver nitrate that were wound at speeds of 175 and 300 ft/min. Stabilization All of the fiber samples were initially stabilized at Clemson University. However, since the Conoco pitch fibers were not sufficiently stabilized, they had to be stabilized further at Chungnam National University. In addition to the stabilization periods mentioned, all of the Conoco as-spun isotropic pitch fibers were exposed to a stabilizing temperature of 230°C for half an hour, then to a temperature of 270ºC for 2 hours and finally to a temperature of 300ºC for 3 hours, all at an elevation rate of 5ºC/min (See Figure 1). Time (hrs) Figure 1. Further stabilization done on all of the Conoco fiber samples. Carbonization The carbonization procedure at Chungnam National University was similar to that at Clemson University. However, the carbon yield percentage was calculated using the following equation: Activation Activation is usually accomplished by heat treating the carbonized fibers with steam or carbon dioxide for a short period. It is at this stage that the ACFs become highly microporous. Ideally, the pore structure connects to the external surface of the fiber, and the pores have diameters ranging from 5 to 21 A [2]. Donnet states that the fraction of pores accessible in the ACF is dependent on the final temperature of the carbonization process and the activation conditions [4]. Previous research indicated that activation with steam is more effective than activation with carbon dioxide. An increase in microporosity is more likely if steam is utilized [5]. Thus, all of the carbonized fiber samples were activated with a mixture of steam and nitrogen in the ratio of 0.455. The steam flow rate was 506ml/min and the nitrogen flow rate was 111ml/min. The carbonized fibers were activated for 10 min, 20 min, and 30 min at a temperature of 900ºC. Surface Studies The Brunauer, Emmett and Teller (BET) analyzer was used to determine the total surface area of the ACFs. A portion of the ACFs was placed into the BET for three hours for degassing. The purpose of this step is to eliminate any impurities that may be present in the fiber. They were then submerged in liquid nitrogen to determine their adsorption capacity. The BET analyzer computed desorption, volume adsorption, and median pore size for each ACF sample and presented the results in graphical form. The results of this surface study were subsequently compared. Results and Discussion 1. Experiments Conducted at Clemson University, Clemson SC, U.S.A Rheology Rheology testing was performed to determine the effect that shear rate had on the flow behavior of the various isotropic pitch samples. From the data obtained, viscosity/shear rate graphs were produced. These graphs suggest possible spinning temperatures for the pitch samples. Figure 2a suggests that the optimum spinning temperature for the unmixed Conoco pitch should lie between 235°C and 250°C. Figure 2b shows that the optimum spinning temperature for the Korea pitch containing 1% silver nitrate is 290ºC or higher. Figure 2c suggests that the spinning temperature of the Conoco pitch containing 1% silver nitrate should be about 245°C. As a check on the reliability of the RDS data, a graph of log (viscosity) was plotted against 1/absolute temperature (Figure 2d). The resulting linear graph proves that the viscosity data are reliable. Spinning Mixed and unmixed samples of each pitch were melt-spun to form fibers with round and trilobal cross sections. The samples with silver nitrate in the pitch required spinning temperatures that were about 10-15ºC higher than the unmixed samples. The trilobal fibers required spinning temperatures that were 5-15ºC lower than those used to form round fibers. The probable cause for this is that the larger cross-sectional area of the trilobal capillaries results in a lower extrusion pressure. This, in turn, leads to a lower spinning temperature. The mixed isotropic pitch samples were more difficult to spin than the corresponding unmixed samples. The conditions used to melt spin round fibers and trilobal fibers (from all pitch samples) are shown in Tables 3 and 4, respectively. SEM pictures of the round and trilobal fibers are shown in Figures 3 and 4.

Figure 2. RDS test results: (a) Log viscosity vs. shear rate of unmixed for Conoco precursor pitch, (b) Log viscosity vs. shear rate for mixed Korea precursor pitch, (c) Log viscosity vs. shear rate for mixed Conoco precursor pitch, (d) Log viscosity vs. reciprocal temperature for raw Conoco precursor pitch.

Figure 3. SEM pictures of the round Conoco fibers: (a) and (b), mixed (1%) fibers spun at 940 ft/min; (c) and (d), unmixed fibers spun at 1500 ft/min.

Figure 4. SEM Pictures of trilobal Korea fibers: (a), mixed fibers spun at 940 ft/min; (b) and (c), mixed fibers spun at 1500 ft/min; and (d), unmixed fibers spun at 1500 ft/min.

Table 3. Melt spinning conditions used to produce round fibers. Pitch (Type) Winding speed (ft/min) Temp. (ºC) Ram speed (RPM) Pressure (psi) Unmixed Conoco 940 245 1700 1320 Unmixed Conoco 1500 245 1700 1320 Unmixed Conoco 2100 245 1700 1320 Unmixed Korea 940 271 1720 325(+) Unmixed Korea 1500 271 1720 325(+) Unmixed Korea 2100 271 1720 325(+) Mixed Conoco 940 261-265 1745 1200 Mixed Conoco 1500 261-265 1690 1500 Mixed Conoco 2100 261-265 1450-1575 1550 Mixed Korea 940 290-306 950-1700 800-2000 Mixed Korea 1500 290-292 820-1550 2200-3000(+) Mixed Korea 2100 292 1300 3000(+) Table 4. Melt spinning conditions used to produce trilobal fibers. Pitch (Type) Winding speed (ft/min) Temp. (ºC) Ram speed (RPM) Pressure (psi) Conoco Unmixed 940 245-250 1680-1720 200-300 Conoco Unmixed 1500 251-253 1725 250 Conoco Unmixed 2100 250 1740 250 Korea Unmixed 940 265-267 1731-1741 190 Korea Unmixed 1500 254-266 1750 190 Korea Unmixed 2100 256-264 1750 200 Conoco Mixed 940 259-266 1155-1580 200-225 Conoco Mixed 1500 269 1580 250 Conoco Mixed 2100 270-271 1705-1710 260 Korea Mixed 940 265 1450 210 Korea Mixed 1500 265 1463-1473 200 Korea Mixed 2100 264 1467 200 2. Experiments Conducted at Chungnam National University, Taejon, Korea Carbonization Table 5. A summary of the carbonization yield for the isotropic pitch fibers. Sample Carbonization Weight before (g) Weight after (g) Yield (%) KOM 940 1.10 0.70 64 COM 940 1.30 0.90 69 KOM 1500 2.10 1.00 48 KOU 1500 1.10 0.80 73 KOM 2100 1.00 0.70 70 KOU 2100 0.20 0.10 50 KOU 1720 1.70 1.00 59 COU 1720 1.20 0.90 75 COM 1500 0.70 0.50 71 COU 1500 2.30 1.70 74 COM 2100 1.70 1.00 59 COU 2100 1.60 1.00 63 Activation The percent burn off for the unmixed samples was found to be higher than the burn off for the mixed samples (see Tables 6 and 7). This was unexpected since past research indicated that the opposite trend should occur. As expected, higher winding speeds yielded fibers with smaller diameters. Comparing samples with different diameters shows that the burn off percent increases as the fiber diameter decreases. This is significant and is most likely due to the higher surface area of the smaller diameter fibers. Table 6. A summary of the burn off percent of the Korea round fibers with time. Sample Time: 10 minutes Time: 30 minutes Burn (%) Yield (%) Burn (%) Yield (%) KOM 175 (3%) 20 80 33 67 KOM 940 20 80 45 55 KOM 300 (3%) 30 70 50 50 KOM 1500 22 78 48 52 KOU 1500 26 74 50 50 KOU 1720 34 66 54 46 KOM 2100 30 70 57 43 KOU 2100 42 38 69 31 Table 7. A summary of the burn off percent of the Conoco round fibers with time. Sample Time: 10 minutes Time: 30 minutes Burn (%) Yield (%) Burn (%) Yield (%) COM 1500 24 76 48 52 COU 1500 28 72 56 44 COU 1720 30 70 58 42 COM 2100 26 74 50 50 COU 2100 39 61 65 35 Surface Studies The surface study of the ACFs was conducted using a Micromeritics® BET analyzer (model ASAP 2010). The activated fibers containing silver nitrate had higher surface areas than those formed from the raw pitch precursor, as shown in Table 8. The silver particles are known to gasify carbon during activation. This increases porosity and, therefore, increased the total surface area of the ACFs. Increasing the silver nitrate from 1% to 3% did not appear to significantly increase the surface area, pore volume, or the median pore diameter. This may be due to the tendency of excess silver particles to escape the fiber during high temperature activation. Table 8. A summary of the surface characterization of the ACFs. Sample BET Data BET surface area (m2/g) Maximum pore volume (cm3/g) Median pore diameter (Å) KOM 175 (3%) 750.3 0.3366 7.8637 KOM 940 2544.9 1.0213 9.2671 KOM 300 (3%) KOM 1500 1835.9 0.7456 8.0854 KOU 1500 1511.9 0.6724 9.9095 KOU 1720 1931.4 0.7998 8.1037 KOM 2100 KOU 2100 COM 940 1608.6 0.6635 8.1805 COM 1500 2263.8 0.937 8.2035 COU 1500 2043.7 0.913 9.2001 COU 1720 2408.7 1.0277 14.6768

Figure 5. Pore size distribution of ACFs based on the HK method.

The desorption isotherm graphs (Figure 5a and 5b.) indicate that the ACFs contain, primarily, micropores. The presence of metals in ACFs gives rise to the occurrence of uniform porosity. In ideal cases, mesoporosity should exist in metal-containing ACFs. However, in this study uniform microporosity was found. The median pore volume for the ACFs formed from both the mixed and unmixed Korea and Conoco pitches was 10Å, as shown in Figures 5c and 5d. Conclusions The mixed ACFs had higher surface areas than the unmixed ACF samples. Therefore, the presence of the silver nitrate did increase the adsorption capability of the carbon fiber. The presence of metals in ACFs gives rise to increased uniformity in the pore size distribution. Although the ACFs produced in this research exhibited uniform microporosity, the pore sizes were much smaller than anticipated. Nevertheless, the maximum measured surface area of 2545 m2g is comparable to the best values reported by other researchers [6]. Works Cited [1] Brasquet, C., Rousseau, B., Estrade-Szwarckopf, H., and Le Cloirec, P. (2002). Observation of Activated Carbon Fibers with SEM and AFM Correlation with Adsorption Data in Aqueous Solution. Journal of Carbon, 38: 407-422. [2] Ryu, S.K., Kim, S.Y., Gallego, N., and Edie, D.D. (1999). Physical Properties of Silver-Containing Pitch-based Activated Carbon Fibers. Journal of Carbon, 37: 1619-1625. [3] El-Merroui, M., Tamai, H., Yasada, H., Kanata, T., Mondori, J., and Nadai, K. (1998). Pore Structures of Activated Carbon Fibers from Organicmetallics/Pitch Composites by Nitrogen Adsorption. Journal of Carbon, 26: 1769-1776. [4] Donnet, J. B., Qin, R.Y., Ryu, S. K., Park, S.J.and Rhee, B. S. (1993). Study of Scanning Tunneling Microscopy on Activated Carbon Fibers. Journal of Material Science, 28: 2950-54. [5] Ryu, S.K., Dondy, D., Pusset, N., Ehrburger, P., and Jin. H. (1993). Activation of Carbon Fibres by Steam and Carbon Dioxide. Journal of Carbon 31 (5): 841-842. [6] Ryu, S.K., Cho, T.H., and Edie, D. (2002). The Distribution of Silver Particles in Silver-Containing Activated Carbon Fibers. Submitted to the Journal of Carbon.