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Langmuir 2006, 22, 702-708
Modeling Water Adsorption in Carbon Micropores: Study of Water in Carbon Molecular Sieves S. W. Rutherford* Los Alamos National Laboratory, Engineering Sciences and Applications DiVision, MS E581, Los Alamos, New Mexico 87545 ReceiVed July 6, 2005. In Final Form: September 16, 2005
Measurements of water adsorption equilibrium in a carbon molecular sieve are undertaken in order to gain insight into the nature of water adsorption in carbon micropores. The measurements are taken at low concentrations to emphasize the role of oxygen-containing functional groups in the adsorption of water. Comparisons are made with previously published water adsorption data at higher concentrations to provide a data set spanning a wide range of loading. The assembled data set provides an opportunity for comparison of various theories for prediction of water adsorption in carbon micropores. Shortcomings of current theories are outlined, and an analytical theory that is free of these deficiencies is proposed in this investigation. With the consideration of micropore volume and pore size distribution, the experimental data and proposed isotherm model are consistent with previous studies of Takeda carbon molecular sieves. Also investigated is the uptake kinetics of water, which is characterized by a Fickian diffusion mechanism. The Maxwell-Stefan formulation is applied to characterize the dependence of the diffusional mobility upon loading.
1. Introduction Microporous carbon is a material that plays a key role in many commercially important technologies ranging from catalyst support, adsorbent, membrane, electrolytic material, abrasionresistant material in magnetic storage media, and mechanical reinforcement material in composites. Recently, new forms of microporous carbon have been derived, including specialty fibers, nanotubes, nanohorns, and buckminsterfullerenes.1 The potential impact of these microporous materials on existing and emerging technologies provides an enormous opportunity for research and development. Some research efforts have been directed toward an understanding of the interaction of these carbon-based materials with their gaseous environments because this interaction can affect performance. Water, in particular, is a common component that interacts with microporous carbon in many of these applications. However, the interaction of water molecules and carbon surfaces is a phenomenon that is still not well understood.2,3 For example, many theories have been proposed to characterize the adsorption equilibrium of water in microporous carbon, but shortcomings are evident when applied to experimental data.4 It is the purpose of this investigation to highlight the strengths and limitations of these theories and to offer a model that is free of the proposed deficiencies. The model is applied to experimental water adsorption data measured on carbon molecular sieves (CMS) manufactured by the Takeda chemical company. This material is chosen because it is a well-studied microporous carbon with a well-defined pore size distribution (PSD). The measurements are taken at low pressure to emphasize the role of oxygencontaining functional groups in the adsorption of water. Comparisons are made to higher-pressure water adsorption data * E-mail:
[email protected]. Phone: (505) 6640812. Fax: (505) 6640815. (1) Carbon Materials for AdVanced Technologies; Burchell, T. D., Ed.; Pergamon: New York, 1999. (2) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity, 2nd ed.; Academic Press: New York, 1982. (3) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999. (4) Brennan, J. K.; Bandosz, T. J.; Thomson, K. T.; Gubbins, K. E. Colloids Surf., A 2001, 187-188, 539-568.
obtained in other investigations in order to provide an isotherm model that can characterize equilibrium over a wide range of concentration and that is consistent with previous studies of Takeda carbon molecular sieves.
2. Background Carbon molecular sieves are useful materials in the gas separation and purification industry because they have finely tuned pore size distributions that allow them to selectively adsorb molecules of size smaller than the pore width while excluding molecules of larger size.5,6 Molecular sieves used for the separation of air can exclude nitrogen molecules (of kinetic size 0.364 nm) from the micropores while allowing oxygen molecules (of kinetic size 0.346 nm) to penetrate. It has been proposed that a pore mouth barrier is responsible for the size exclusion effect.7 In the case of CMS used in air separation, known as CMS 3A, the size of this barrier is expected to be on the order of 3 Å. According to Juntgen et al.,8 hydrocarbon cracking is the crucial step in the manufacture of these materials that yields the pore mouth barrier. The carbon that is deposited from this cracking step is assumed to reside at the entrance of the micropore and not significantly penetrate the micropore itself.7 Under this assumption, the pore size distribution of the carbon would be essentially unaltered by the cracking treatment. A reduction may be evident in the measured total micropore volume per gram of sample because the deposited carbon adds additional weight to the sample. However, this has not been widely investigated because of the difficulty posed by CMS 3A in applying the standard technique of pore size determination using nitrogen adsorption at 77 K. At 77 K, nitrogen molecules fail to penetrate the material on a reasonable time scale, thereby rendering the technique unusable. Nevertheless, Takeda CMS 3A and CMS (5) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984. (6) Yang, R. T. Adsorbents: Fundamentals and Applications, Wiley: New York, 2003. (7) Karger, J.; Ruthven, D. M. Diffusion in Zeolites and Other Microporous Solids; Wiley: New York, 1992. (8) Juntgen, H.; Knoblauch, K.; Harder, K. Fuel 1981, 60, 817.
10.1021/la051826n CCC: $33.50 © 2006 American Chemical Society Published on Web 12/15/2005
Study of Water in Carbon Molecular SieVes
Figure 1. Corrected Dubinin-Radushkevich plot of the adsorption of carbon dioxide in Takeda 3A and Takeda 5A at 20 °C.
5A, which are employed for the separation of larger molecules on the order of 5 Å, have been studied by other techniques and appear to display differing total micropore volumes. Horvath and Kawazoe9 calculated the PSD for Takeda 5A from measurements of nitrogen adsorption. The PSD was found to have a modal value at around 0.5 nm and negligible micro/mesopores of size greater than 1 nm, a result later verified by methyl chloride adsorption.10 Rutherford et al.11 calculated a similar PSD for Takeda 3A from methane adsorption data and obtained a modal value at 0.5 nm and negligible micro/mesopores of size greater than 1 nm. Additionally, Cazorla-Amoros et al.12 obtained carbon dioxide isotherms for Takeda 5A and 3A and showed a larger saturation capacity for Takeda 5A at around 6.9 mmol/g compared to 4.2 mmol/g for Takeda 3A. This corresponds to a total micropore volume of 0.31 (cm3 of micropore volume)/g for Takeda 5A and 0.19 (cm3 of micropore volume)/gram for Takeda 3A (using a carbon dioxide density of 1.023 g/cm3 12). Using these values, the data for carbon dioxide adsorption in Takeda 3A and 5A obtained by Rutherford and Do13,14 can be recalculated on the basis of the amount adsorbed per cubic centimeter of micropore volume (Cµ). The Dubinin-Radushkevich plot, which is commonly employed to obtain micropore size distributions, presents the recalculated data in Figure 1. It can be seen that the data appear to overlay to provide a single curve, thereby generating further evidence that the micropore size distributions for the two materials are similar. 2.1. Surface Chemistry of Takeda CMS. Technical information from the manufacturer indicates that both Takeda 3A and 5A CMS are derived from coconut shell, which provides a useful precursor material for CMS manufacture. On a molecular level, coconut shell-derived carbon has graphite-like pores that have surface corregations. Turner, Pikunic, and Gubbins15 supply a snapshot of the structure from an atomistic standpoint and derive a PSD very similar to that obtained for both Takeda 3A and 5A (i.e., micropores smaller than 1 nm and a modal value of 0.5 nm). Also included in the molecular model of coconut shell-derived (9) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470-475. (10) Mariwala, R. K.; Foley, H. C. IEC Res. 1994, 33, 2314-2321. (11) Rutherford, S. W.; Nguyen, C.; Coons, J. E.; Do, D. D. Langmuir 2003, 19, 8335-8342. (12) Cazorla-Amoros, D.; Alcaniz-Monge, J.; de la Casa-Lilli, M. A.; LinaresSolano, A. Langmuir 1998, 14, 4589-496. (13) Rutherford, S. W.; Do, D. D. Carbon 2000, 38, 1339-1350. (14) Rutherford, S. W.; Do, D. D. Langmuir 2000, 16, 7245-7254. (15) Turner, C. H.; Pikunic, J.; Gubbins, K. E. Mol. Phys. 2001, 99, 19912001.
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carbon are oxygen-containing functional groups that may include carboxylic groups (as considered by Turner et al.15), but there are also significant carbonyl and hydroxyl groups in coconut shell-derived carbon.16 The functional groups are believed to be located at the edges of the graphene layers that stack to form the micropore structure in the CMS.17 Hence, these sites provide an opportunity for water to bond outside of the stacked graphene structure. Additionally, it has been shown by molecular simulation that water may also adsorb within the micropores formed from the spacing between the stacked graphene layers that offer attraction for the water molecules. In fact, it is shown that small micropores offer a larger attraction for water than do larger micropores.18,19 Molecular simulations also show that a threshold of 1 nm exists where pores smaller than this size can accommodate only a single layer of water molecules.18,19 Applied to Takeda 3A and 5A CMS, this would imply that monolayer adsorption of water occurs within these materials. Due to the similar PSD for Takeda 3A and 5A and the expectation of simple monolayer water formation, the molecular sieves provide useful materials to experimentally and theoretically study the phenomenon of water adsorption in micropores. Water molecules are also useful probes in assessing ultramicroporosity in carbon molecular sieves due to small molecular size that allow them to penetrate micropores without interference from the pore mouth barrier of CMS.
3. Water Adsorption in Takeda 5A Water adsorption in Takeda 5A samples has been measured by volumetric adsorption conducted differentially as is outlined in a previous investigation.20 The Takeda 5A samples have the following characteristics: the pellets are 0.3 cm in diameter and 0.3 cm in length and have been characterized by mercury porosimetry yielding a total macroporosity at 28% and an average macropore size on the order of 0.5 micron.13 The measurement of the adsorption equilibrium of water in these pellets was undertaken at 20 °C from six independent experiments separated by an outgassing period of 72 h under ultrahigh vacuum at 20 °C. The results are shown in Figure 2 for relative pressures up to 0.4, a range that can allow elucidation of the role of oxygen containing functional groups in the adsorption of water. The isotherm plot in Figure 2 shows an observable rise at very low loadings and could be considered to be of type IV in the BDDT classification scheme.21 Evans22 proposes that this rise at low loadings is difficult to observe unless careful measurement of data at low pressure is made. This is due to the upturned nature of the isotherm which leads to much larger amounts adsorbed at higher pressures, allowing the lowpressure data to be obscured due to scaling. Additionally, Evans22 attributes the behavior at low loading to a Langmuir-type binding of water molecules to the functional groups within the carbon. Also evident from Figure 2 is the fact that the equilibrium measurements were conducted in adsorption and desorption modes. It appears that there is a negligible difference between points obtained from adsorption and desorption in the range (16) Albers, P. W.; Pietsch, J.; Krauter, J.; Parker, S. F. Phys. Chem. Chem. Phys. 2003, 5, 1941-1949. (17) Marsh, H., Ed. Introduction to Carbon Science; Butterworths: Cornwall, England, 1989. (18) Striolo, A.; Chialvo, A. A.; Cummings, P. T.; Gubbins, K. E. Langmuir 2003, 19, 8583-8591. (19) Striolo, A.; Gubbins, K. E.; Chialvo, A. A.; Cummings, P. T. Mol. Phys. 2004, 102, 243-251. (20) Rutherford, S. W.; Coons, J. E. Langmuir 2004, 20, 8681-8687. (21) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723. (22) Evans, M. J. B. Carbon 1987, 25, 81-83.
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Figure 2. Water adsorption and desorption equilibria of Takeda 5A CMS at 20 °C.
Figure 3. Water adsorption equilibria of Takeda 3A and Takeda 5A at 20 °C.
studied, indicating negligible hysteresis. The lack of observable hysteresis is significant because it precludes the intrusion of capillary condensation, which represents a different mechanism to the molecular-level phenomena investigated in this study. A similar observation was made for water adsorption in Takeda 3A by Rutherford and Coons.20 In contrast to this result are simulation studies that propose that hysteresis should be observed even in pores of size 1 nm.18,19 The discrepancy may be attributable to the difference between the experimental and simulation conditions. The simulation studies18,19 involved loading of water into the micropores to complete filling where hysteresis effects would be more pronounced. The experiments of this investigation involve water loading to less than approximately one-third of the saturation capacity. Under these conditions, the effects of molecular packing that are assumed to be responsible for hysteresis may be less pronounced. Further simulation studies that involve partial loading and unloading of the micropores are required to resolve the apparent discrepancy. As a measure of validation, the data obtained in this investigation is compared to the data obtained for Takeda 3A in a previous investigation.20 Figure 3 shows the amount of water adsorbed, expressed on the basis of the amount adsorbed per cubic centimeter of micropore. The water adsorption data for
Rutherford
Figure 4. Water adsorption equilibria on Takeda 3A and Takeda 5A together with the fit of the composite Langmuir-Ising model, represented by eq 3. The contribution of the Ising equation is also shown.
Takeda 3A and 5A appears to form a single curve, as was the case for carbon dioxide adsorption. This result indicates that consistent behavior is observed for both water adsorption and carbon dioxide adsorption in Takeda CMS. Furthermore, these data can be coupled with the data obtained by Alcaniz-Monge et al.23 at higher pressures and at a higher temperature as shown in Figure 4. From a theoretical standpoint, the temperature dependence of the isotherm is expected; however, we note that the saturation capacity and therefore the data at loadings approaching saturation should be approximately independent of temperature over a range of 5 °C. Additionally, the data presented by Alcaniz-Monge23 at low loading is obscured by a large number of measurements for other carbon materials and the Takeda CMS data cannot be accurately reproduced at low loading. For these reasons, only the data at higher loading are included in Figure 4. It can be seen that there is some deviation between the data of this investigation and that of Alcaniz-Monge et al.23 at mid to high loading of water. Such differences are ascribed to variations in outgassing conditions that can have a significant effect on the water adsorption isotherm according to AlcanizMonge et al.23 Nevertheless, the combination of data from various sources provides a data set spanning almost a 1000-fold change in relative pressure. This data set provides an opportunity for a comparison of various theories for prediction of water adsorption in carbon micropores. These theories are considered in the following section.
4. Water Adsorption Isotherm Equations Water adsorption in microporous carbon is known to display complex behavior because of the low energy of interaction with the carbon surface coupled with the high potential for association of water molecules. This complex behavior leads to a variety of observed isotherm types ranging from type I to type V of the BDDT21 classification scheme. Obviously, a robust model for water adsorption must be capable of characterizing this widely varying behavior. Specifically, the following criteria for a robust equilibrium isotherm model are proposed: (1) Water adsorption on carbon surfaces with a relatively low affinity for water, for example, graphite surfaces, display type (23) Alcaniz-Monge, J.; Lozano-Castello, D. Adsorpt. Sci. Technol. 2003, 21, 841-848.
Study of Water in Carbon Molecular SieVes
III/V behavior. Water adsorption in microporous carbons with measurable oxygen content appear to display significant amounts adsorbed at very low pressure (
