Activated Carbon Monoliths from Phenol Resin and Carbonized

Energy & Fuels 2005, 19, 251-257

251

Activated Carbon Monoliths from Phenol Resin and Carbonized Cotton Fiber for Methane Storage Akinori Muto, Thallada Bhaskar, Shinichiro Tsuneishi, and Yusaku Sakata* Department of Applied Chemistry, Faculty of Engineering, Okayama University, 3-1-1 Tsushima Naka, Okayama 700-8530, Japan

Hiroshi Ogasa Honda R & D Co., Ltd., Tochigi R & D Center, 4630 Takanezawa, Haga-cho, Haga-gun, Tochigi 321-3393, Japan Received March 3, 2004. Revised Manuscript Received September 9, 2004

Adsorbed natural gas has various advantages and is relatively more economical than liquefaction and compression. In our present investigation, we have prepared the carbon monolith from a phenol resin mixed with a renewable resource (such as cotton fiber) after carbonization. The compositions were optimized to determine the highest V/Vs value with methane gas and achieved a value of 140 cm3/cm3 (at 35 °C and 3.5 MPa) with carbon monolith prepared from a 75:25 mixture of phenol resin and cotton fiber. The effects of various physical properties on the methane adsorption capacity were studied in detail. The use of carbonized cotton fiber increased the methane adsorption, surface area, and pore volume. Characterization of the carbon monoliths were performed using the Brunauer-Emmett-Teller (BET) surface area, and the pore size distribution, and the morphology was examined via scanning electron microscopy (SEM).

Introduction Natural gas has considerable advantages over conventional fuels, both from an environmental point of view and for its natural abundance. It is well-known that its great disadvantage is its lower heat of combustion per unit of volume, when compared with conventional fuels.1 Compressed natural gas (CNG) may be an alternative solution; however, high pressures are needed (up to 25 MPa) for use in vehicles fueled by natural gas. The large cost of the cylinders used for storage and the high-pressure facilities that are necessary limit the practical use of CNG.2 Alternatively, adsorbed natural gas (ANG), at pressures up to 4 MPa, offers a very highpotential alternative for use in both transport and largescale applications. However, its actual equivalent storage, in comparison to that of CNG, is quite low (8-10 MPa); thus, the development of a suitable adsorbent is necessary to maximize the methane uptake per unit of storage volume. Among the available adsorbents, activated carbons exhibit the largest adsorptive capacity.2,3 The recent development of high-capacity adsorbents has produced a vast amount of literature,4-11 showing * Author to whom correspondence should be addressed. Telephone: +81 86 251 8081. Fax: +81 86 251 8082. E-mail: yssakata@ cc.okayama-u.ac.jp. (1) Eberhardt, J. J. Gaseous Fuels in TransportationsProspects and Promise; Presented at the Gas Storage Workshop, Kingston, Ontario, Canada, July 10-12, 2001; Sponsored by the Royal Military College of Canada and the American Carbon Society. (2) Parkyns, N. D.; Quinn, D. F. In Porosity in Carbons: Characterization and Applications; Patrick, J. W., Ed.; Edward Arnold: London, 1995; p 302. (3) Cracknell, R. F.; Gordon, P.; Gubbins, K. E. J. Phys. Chem. 1993, 97, 494.

the rapid growth and importance of the subject. Highpressure adsorption data were obtained for methane, ethane, ethylene, propane, carbon dioxide, and nitrogen on activated carbons. Adsorption isotherms that were obtained up to 6 MPa and at various temperatures were measured, and the isosteric heats of adsorption were determined from adsorption isosteres by the ClausiusClapeyron equation.4 Kaneko et al.5 produced nanostructured disordered carbon, and it exhibited a methane storage capacity of 160 cm3/cm3 at 3.5 MPa and 303 K. Comparison of the experimental results with Grand Canonical Monte Carlo simulations indicated the importance of the adsorption in the interstitial channels for the high total adsorption capacity of the carbons.5 The Grand Canonical Monte Carlo simulations were performed for natural gas that had been adsorbed on carbon. Natural gas was modeled as pure methane adsorbed on parallel planes of graphite. Comparison of the molecular simulations with experimental data showed that, at pressures of >1 MPa, the slit model provides an upper bound for equilibrium capacity.6 (4) Agarwal, R. K.; Schwarz, J. A. J. Colloid Interface Sci. 1989, 130, 137. (5) Bekyarova, E.; Murata, K.; Yudasaka, M.; Kasuya, D.; Iijima, S.; Tanaka, H.; Kahoh, H.; Kaneko, K. J. Phys. Chem. B 2003, 107, 4681. (6) Matranga, R. K.; Myers, A. L.; Glandt, E. D. Chem. Eng. Sci. 1992, 47, 1569. (7) Burchell, T.; Rogers, M. SAE Tech. Pap. Ser. 2000, 2000-01-2205. (8) Lozano-Castello, D.; Alcaniz-Monge, J.; De La Casa-Lillo, M. A.; Cazorla-Amoros, D.; Linares-Solano, A. Fuel 2002, 81, 1777. (9) Alicaniz-Monge, J.; De La Casa-Lillo, M. A.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 1997, 35, 291. (10) MacDonald, J. A. F.; Quinn, D. F. Fuel 1998, 77, 61. (11) Perrin, A.; Celzard, A.; Mareche, J. F.; Gurdin, G. Energy Fuels 2003, 17, 1283.

10.1021/ef0400316 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/26/2004

252

Energy & Fuels, Vol. 19, No. 1, 2005

Methane (the major constituent of natural gas) has a higher H/C ratio than any other fuel and, consequently, a higher researched octane number than other fuels (130, compared to 87 for unleaded gasoline).7 Although, in principle, any material with high adsorption capacity can be considered for bulk storage of natural gas, of the four major industrial types of adsorbents, i.e., zeolites (natural and synthetic), silica gel, activated alumina, and activated carbon, the latter category is, by far, bestsuited for such applications. This is due to the unique surface property of activated carbon being nonpolar: it adsorbs more nonpolar or weakly polar organic molecules than other types of adsorbents. As a consequence, the methane carbon strengthsand, therefore, the heat of adsorptionsgenerally are lower than those values for other adsorbents, resulting in lower energy requirements for the regeneration process. Unfortunately, methane cannot be stored at a density as high as other fuels; thus, it has an energy density that is ∼1/3 that of gasoline (11 MJ/L for CNG at 24.8 MPa (3600 psi), compared to 32 MJ/L for gasoline). Thus, a CNG fuel tank would need to be ∼3 times larger than a gasoline tank to allow a vehicle the same driving range. The use of CNG has its disadvantages: e.g., CNG storage tanks must be pressure vessels and, thus, are constrained in their geometry (they are typically cylindrical), and they are rather heavy (1 kg/L for steel tanks). Moreover, attainment of a pressure of >20.7 MPa (3000 psi) requires costly multistage compression.7 The choice of a suitable type of carbon is one of the key factors in determining the overall economics of an ANG scheme, and the screening and selection process is most essential in this research field. For these reasons, the U.S. Department of Energy (DOE) has pursued a research program aimed at the development of suitable materials for the storage of natural gas in the physically adsorbed state. ANG is conventionally stored in porous carbon materials at a gas pressure of 3.5 MPa (500 psi). This lower storage gas pressure reduces the cost of the storage vessel, allows the use of single-stage compressors, and represents a lesser safety hazard than the higher pressure used for CNG. The DOE storage target for ANG has been set at 150 V/V (i.e., 150 L of gas (at STP, 101.325 kPa and 298 K) stored per liter of pressure vessel internal volume).7 In the present investigation, we report the preparation of disk-type carbon monoliths from phenol resin and cotton fiber with various compositions to achieve a high-methane-storage-capacity carbon monolith at low pressures and temperature. The effects of various physical parameters, optimization of the phenol resin and cotton fiber composition, and the activation time were investigated. Experimental Section Preparation of Carbon Monolith and Activation. The carbonized cotton fiber was obtained from Tokai Senko K.K., Nagoya, Japan (surface area of 960 m2/g, pore volume of 0.53 mL/g). The phenol resin was purchased from Kanebo, Ltd., Japan. The required amount of phenol resin and carbonized cotton fiber was mixed thoroughly and placed into the cell to obtain the disk-type carbon monolith. The disk-type carbon monolith was prepared using high pressure and high-temperature electric furnace under the inert gas (nitrogen) at 900 Kgf/cm2 until it reached a temperature of 440 °C. The detailed temperature program for the preparation of the carbon mono-

Muto et al. lith is presented in Figure 1a. Furthermore, the disk-type monolith was activated using CO2 at 800 °C with a flow rate of 300 cm3/min. The conventional downflow fixed-bed reactor (which was composed of Pyrex glass) was inserted into the laboratory-made high-temperature furnace, and the activation was conducted using CO2. Measurement of Methane Adsorption Capacity. The schematic experimental setup used for the methane gas adsorption studies were presented in Figure 1b. An activated disk-type carbon monolith was loaded into the adsorption cell, and the adsorption capacity of methane was measured at various pressures in the range of 0.35-4.0 MPa. The sample cell was evacuated and kept at 110 °C for 1 h, and the temperature then was reduced by cooling. The complete adsorption setup was kept in a water bath. The pressure-volume relationships were studied under the equilibrium conditions, and the results were discussed. Measurement of Real Density. The density measurements for the activated disk-type carbon monolith were obtained using a micropycnometer (Yuasa-Ionics Co., Ltd., Japan). The sample was well-dried in an oven (at 110 °C) for 30 min before the density measurements. The cell weight was measured in the absence and the presence of the carbon sample, after evacuation for a period of 1 h. Helium gas was used, and the density was obtained using the following equations:

density (g/cm3) )

W V

where

V1 ) Vr - Vs

(

P1 -1 P2

)

V1 is the volume of the sample, Vr is the volume of the gas holder, Vs is the sample cell volume, P1 is the initial pressure inside the sample holder, and P2 is the final pressure inside the sample holder. The BET surface area and pore-size distribution (the DH-MP method) of the samples were measured by nitrogen adsorption at 77 K, using BELSORP28 (from Nihon Bell, Japan). The methane adsorption capacity (V/Vs) of carbon monoliths was estimated as the volume of methane adsorbed per the volume of the carbon monolith. (V is a total methane storage volume, and Vs is the vessel volume.)

Results and Discussion Methane Storage Capacity Studies of Carbon Monolith from the Phenol Resin. The characterization data obtained from N2 adsorption/desorption isotherms and the adsorption capacity of carbonized phenol resin with various activation treatments are presented in Table 1. The Brunauer-Emmett-Teller (BET) surface area of the carbon monolith increased from 450 m2/g to 650 m2/g as the activation time increased from 0 h to 7 h and the pore volume of the carbon monolith increased from 0.20 mL/g to 0.31 mL/g. The increase of activation time increased the surface area and pore volume by increasing the porous network, and, subsequently, the shrinkage volume was decreased. The methane adsorption studies were performed at a pressure of 3.5 MPa and a temperature of 35 °C, and the methane adsorption results are given in Table 1. This table shows that the amount of methane adsorption increases as the activation time increases. However, after 5 h of activation, there is no appreciable change in volume shrinkage, surface area, pore volume, and

Activated Carbon Monoliths for Methane Storage

Energy & Fuels, Vol. 19, No. 1, 2005 253

Figure 1. (a) Temperature program for the preparation of carbon monoliths. (b) Schematic experimental setup for the methane adsorption capacity. Table 1. Effect of Activation Time on the Textural Properties of Carbon Membrane from Phenol Resin shrinkage adsorption surface pore activation in volume at 3.5 MPa area volume sample time (h) (cm3/cm3) (cm3(STP)/g) (m2/g) (mL/g) P1 P2 P3 P4 P5 P6

0 1 2 5 7 8

1.00 0.75 0.74 0.73 0.72 0.71

45 80 83 90 95 97

450 500 545 630 650

0.20 0.24 0.26 0.30 0.31

amount of methane adsorption. Beyond the 5 h of activation time, up to 8 h, the shrinkage in volume and adsorption has not increased. Under these circumstances, it can be concluded from Table 1 that the 5 h of activation was sufficient for the effective porous structure, which can adsorb the high amount of methane. The effect of activation time on the methane storage capacity (V/Vs) was presented in Figure 2. The carbonized phenol resin without activation showed a V/Vs value of ∼70. However, the activation time (1 h) improved the V/Vs drastically, to 110. Further increases in the activation time improved the V/Vs, up to an activation time of 5 h; increases in the activation time beyond this time has not given effective V/Vs values. To understand the physical properties of the carbon monolith for the high adsorption capacity, the relationship between the apparent density and the activation

time was established, along with the V/Vs value (Figure 3). As the activation time increased, the apparent density of the carbon monolith decreased, from an activation time of 1 h to 5 h; further increases in the activation time has not decreased the apparent density with appreciable quantity. This phenomenon might be because the porous network was established by the removal of carbon with the 5 h of activation time and it showed V/Vs values of ∼125 (and, with 7 h of activation time, V/Vs ) 128). There has been a plethora of research work on the utilization of activated carbons for methane gas storage. Lozanno-Castello et al.8 reviewed the advances in the study of methane storage in porous carbonaceous materials such as activated carbon fibers, chemically activated carbons, and activated carbon monoliths. Perrin et al.11 compared the methane storage capacities on dry and water-wetted active carbon powder. The amount of gas stored at the highest pressures investigated in that work ranged from 180 V/V to 230 V/V (volume (STP) per unit volume of storage vessel (V/Vs)), depending on the material, whereas only 110-160 V/V are obtained with dry carbons (at 2 °C and 8 MPa).11 The larger the surface area, the higher the methane uptake will be. However, because methane storage (at room temperature and pressures up to 4 MPa) is exclusively restricted to micropore volumes, materials with mesopore and macropore volumes must be avoided.

254


Muto et al.

Figure 2. Effect of activation time (300 mL/min of CO2 at 800 °C) on the methane adsorption capacity of activated carbon monolith obtained from phenol resin.

Figure 3. Effect of activation time (300 mL/min of CO2 at 800 °C) on the apparent density of the carbon monolith.

Methane Storage Capacity of Carbon Monolith Obtained from Mixture of Phenol Resin and Cotton Fiber. The physicochemical properties of a carbon monolith obtained from a mixture of phenol resin and carbonized cotton fiber was presented in Table 2. The carbonized cotton fiber was mixed in various proportions with the phenol resin, and different activation times were applied to prepare several carbon monoliths. Samples PC1 and PC2 in Table 2 were prepared with no activation and with 2 h of activation, which was used for the comparison of mixed carbon monoliths. Samples PC3-PC6 were obtained as the amount of carbonized cotton fiber was increased from 10 wt % to 25 wt % with 2 h of activation. The shrinkage in volume increased as the addition of carbonized cotton fiber increased, up to 20 wt %; no further change was observed with 25 wt % of cotton fiber. However, a small increase in methane adsorption and pore volume, and an increase in surface

Table 2. Effect of Addition of Carbonized Cotton Fiber on the Textural Properties of Activated Carbon Membrane of Phenol Resin resin: shrinkage surface pore activation cotton in volume adsorption area volume 3 3 3 sample time (h) mixture (cm /cm ) (cm (STP)/g) (m2/g) (mL/g) PC1 PC2 PC3 PC4 PC5 PC6 PC7 PC8 PC9 PC10

0 2 2 2 2 2 5 5 0 7

100:0 100:0 90:10 85:15 80:20 75:25 80:20 75:25 70:30 75:25

0.74 0.77 0.78 0.84 0.85 0.75 0.74

45 83 110 115 120 125 180 245

450 545 690 760 860 950 1240 1440

0.20 0.26 0.37 0.40 0.44 0.58 0.85 0.93

We could not obtain a carbon membrane

area (from 690 m2/g to 950 m2/g), was observed when the mixture ratio of resin to cotton changed from 90:10 to 75:25. As was observed from Table 1, an activation time of ∼5 h was produced the higher V/Vs values;



Figure 4. Pore size distribution analysis of carbonized cotton fiber (as received) and the carbon monolith that was composed of a mixture of phenol resin and carbonized cotton fiber (75:25) for activation times of 2 h and 5 h.

Figure 5. Methane adsorption capacity of various proportions of carbonized cotton fiber and phenol resin with different activation times.

therefore, in the mixed monolith, we also applied an activation time of 5 h with the resin:activated cotton fiber ratios of 80:20 and 75:25. Although there is no appreciable change in the shrinkage of volume, the adsorption capacity and the surface area and pore volume increased dramatically. The 80:20 ratio of resin and carbonized cotton fiber exhibited an adsorption of 120 cm3/g with a surface area of 860 m2/g for an activation time of 2 h; however, it exhibited an adsorption of 180 cm3/g when the surface area was 1240 m2/g for an activation time of 5 h. In a similar way, a 75:25

resin:activated cotton fiber ratio exhibited an adsorption of 125 cm3/g, with a surface area of 950 m2/g for an activation time of 2 h and it increased to 245 cm3/g with a surface area of 1440 m2/g for an activation time of 5 h. The drastic increase in surface area and adsorption values might be due to the development of a micropore structure in the carbon monolith, and it was analyzed by nitrogen adsorption/desorption isotherms. The representative pore size distribution data for samples PC6 and PC8 and the carbonized cotton fiber were measured, and the results are presented in Figure 4. As can be

256


Muto et al.

Figure 6. Dependence of methane adsorption capacity on (a) the surface area and (b) the pore volume of activated carbon monoliths from various proportions of carbonized cotton fiber and phenol resin.

observed from Table 2 and Figure 4, sample PC6 was obtained with an activation time of 2 h and sample PC8 was obtained with an activation time of 5 h. The increase in activation time from 2 h to 5 h caused the pore volume to double, and the pore size was 0.7 nm. It is evident from the other properties that the increase of activation time for the mixed carbon monolith and resin carbon monolith increased the adsorption properties. The methane storage capacity of carbon monoliths prepared under various conditions were studied and compared in Figure 5. The mixed and activated carbon monolith showed a V/Vs value of 143. Effect of Various Physical Properties on the V/Vs Value. As the amount of added carbonized cotton fiber is increased, the apparent density of the carbon monoliths decreases. We tried to incorporate >25 wt % carbonized cotton fiber, but we could not obtain the carbon monolith and we thought that this limits the mixture of phenol resin with carbonized cotton fiber for the preparation of a carbon monolith. To achieve higher V/Vs values, we have kept the amount of carbonized cotton fiber at 25 wt % and increased the activation time. Kaneko et al.7 reported that the compression of single-wall carbon nanohorns repeatedly at 50 MPa produced material with high apparent density and disordered carbon exhibits a high methane storage capacity, reaching 160 cm3/cm3. The effect of surface area on the methane adsorption capacity was presented in Figure 6a. It is evident from Figure 6a that the increase in surface area increased the methane gas

Figure 7. Scanning electron microscopy (SEM) analysis of representative activated carbon monoliths: (a) carbonized cotton fiber (as received), (b) 25% carbonized cotton fiber and phenol resin activated for 2 h (sample F in Table 2), and (c) 25% carbonized cotton fiber and phenol resin activated for 5 h (sample H in Table 2).

adsorption capacity. However, from the fundamental studies7,8 on the methane storage on carbons, we know that the micropore has a major role in the higher adsorption capacities. The increase in the number of micropores was proportional to the increase in surface area in the carbon monoliths prepared in this study, and the results are presented in Figure 6b. The plot between BET surface area and pore volume showed a linear relationship. The storage capacity is directly linked to the volume of micropores that have the ideal width, and getting high micropore volumes requires an activation step. Now, activation is efficient only if some porosity is already present. That is the reason why pyrolyzed


phenolic resin is not successfully activated. However, adding cellulose and pyrolyzing the entire sample leads to a fine porosity, because the carbon yield of cellulose is low, in comparison to that of the resin. After the former porosity is created, the activating agent can diffuse and create more pores and enlarge the former pores. This is a very classical phenomenon in carbon science. It is very important to optimize the conditions such as carbonization temperature and time, activation temperature, and time to obtain monoliths with suitable mechanical strength. Lozanno-Castello et al.12 studied the influence of binder on the methane storage capacity of activated carbon monoliths. The preliminary studies showed that methane adsorption up to 4 MPa for the monoliths does not present diffusional problems for the adsorption of methane, and the present investigation and characterization results are in good agreement with the morphology; the present study showed higher methane adsorption capacities with the addition of cotton fiber. Under these conditions, the morphology was examined by scanning electron microscopy (SEM) studies, and they are presented in Figure 7. As can be (12) Lozano-Castello, D.; Cazorla-Amoros, D.; Linares-Solano, A.; Quinn, D. F. Carbon 2002, 40, 2817.


seen from these images, the carbonized cotton fiber has a compact structure and, with the addition of phenol resin and an increase in activation time, produced the open structures; Figure 7 also shows that the structure is dependent on the composition. Conclusions The successful development of disk-type carbon monoliths from a mixture of phenol resin and carbonized cotton fiber was performed. The laboratory evaluation studies for methane adsorption studies showed that disk-type carbon monoliths obtained under optimized conditions exhibited a V/Vs value of 140 at 3.5 MPa and 35 °C with marginal yields. The optimized preparation and conditions gave monoliths that had porous open structures with suitable density, which are required for methane storage. Acknowledgment. We thank the anonymous reviewers for their valuable comments and suggestions. We are thankful to Tokai Senko K.K., Nagoya, Japan, for providing cotton samples, and to Mr. T. Toyama, for valuable discussions. EF0400316

Activated Carbon Monoliths from Phenol Resin and Carbonized

Recommend Documents