pubs.acs.org/Langmuir © 2009 American Chemical Society
Fundamentals of High Pressure Adsorption† Yaping Zhou Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, P. R. China
Li Zhou* High Pressure Adsorption Laboratory, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P R China Received June 1, 2009. Revised Manuscript Received August 14, 2009 High-pressure adsorption attracts research interests following the world’s attention to alternative fuels, and it exerts essential effect on the study of hydrogen/methane storage and the development of novel materials addressing to the storage. However, theoretical puzzles in high-pressure adsorption hindered the progress of application studies. Therefore, the present paper addresses the major theoretical problems that challenged researchers: i.e., how to model the isotherms with maximum observed in high-pressure adsorption; what is the adsorption mechanism at high pressures; how do we determine the quantity of absolute adsorption based on experimental data. Ideology and methods to tackle these problems are elucidated, which lead to new insights into the nature of high-pressure adsorption and progress in application studies, for example, in modeling multicomponent adsorption, hydrogen storage, natural gas storage, and coalbed methane enrichment, was achieved.
1. Introduction High-pressure adsorption is synonymous of the adsorption at above-critical temperatures because adsorption pressure cannot be high if the adsorption temperature is lower than critical temperature, due to the limitation of saturation pressure. The adsorbed amount is usually low at above-critical temperatures; therefore, this kind of adsorption did not attract much research interest until the last seventies when the world felt crisis of petroleum supply. Natural gas and hydrogen were considered alternative fuels of petroleum products, and on-board storage of gaseous fuels became a hot topic of research. Adsorption is efficient in increasing gas density; therefore, the adsorption of methane (the major component of natural gas) and hydrogen has received extensive studies since then. High-pressure adsorption is, indeed, the physicochemical basis of many important engineering processes and potential industrial applications; therefore, challenges and opportunities are showing up at the same time in a field of growing importance for both science and engineering. However, there are some theoretical problems in high-pressure adsorption that puzzle researchers. The first theoretical problem in the study of high-pressure adsorption is to explain the experimentally observed adsorption isotherms of methane or hydrogen at ambient temperature. These isotherms may show a maximum and a descending section after the maximum following the increase of adsorption pressure, which cannot be modeled with models available presently for isotherms. Although different types of isotherm models were proposed for high-pressure † This is a revised version of the plenary keynote lecture titled as “Challenges and opportunities in high pressure adsorption” presented at the fifth Pacific Basin Conference on Adsorption Science and Technology, May 25-27 2009, Singapore. *To whom correspondence should be addressed. Telephone and fax: þ86 22 87891466. E-mail:
[email protected].
(1) Ozawa, S.; Kusumi, S.; Ogino, Y. J. Colloid Interface Sci. 1976, 56, 83-91. (2) Czepiriski, L.; Jagiello, J. Chem. Eng. Sci. 1989, 44, 797-801. (3) Malbrunot, P.; Vidal, D.; Vermesse, J.; Chahine, R.; Bose, T. K. Langmuir 1992, 8, 577-580.
Langmuir 2009, 25(23), 13461–13466
adsorption in the literature,1-20 three stages could be identified for the progress of modeling strategy. The first stage is to entail conventional isotherm models with assumptions of quasi quantities, e.g., quasi-liquid, overheated liquid, or quasi-saturation pressure. These models may look successful if the experimental pressure is not high enough to show the isotherm maximum. In fact, all conventional isotherm models are initially developed for absolute adsorption and, therefore, are increasing functions of pressure. They cannot describe decreasing functions as the isotherm after maximum. The second stage is to use a mathematical function that allows for a curve with maximum. The Ono-Kondo equation was shown to be successful, but not for all experimental isotherms. In addition, parameters in the model lack a reasonable physical meaning. The third stage is to use a model with multiple parameters established via EOS (equation of state) or DFT (density function theory). This kind of model satisfactorily models the experimental isotherms of high-pressure adsorption, and the DFT model has been applied in characterization of porous solids. However, there is a theoretical gap between (4) Kaneko, K.; Shimizu, K.; Suzuki, T. J. Chem. Phys. 1992, 97, 8705-8711. (5) Shethna, H. K.; Bhatia, S. K. Langmuir 1994, 10, 870-876. (6) Vermesse, J.; Levesque, D. J. Chem. Phys. 1994, 101, 9063-9071. (7) Jiang, S. Y.; Zollweg, J. A.; Gubbins, K. E. J. Phys. Chem. 1994, 98, 5709-5713. (8) Amankwah, K. A. G.; Schwarz, J. A. A. Carbon 1995, 33, 1313-1319. (9) Subramanian, R.; Pyada, H.; Lira, C. T. Ind. Eng. Chem. Res. 1995, 34, 3830-3837. (10) Aranovich, G.; Donahue, M. J. Colloid Interface Sci. 1996, 180, 537-541. (11) Jensen, C. R. C.; Seaton, N. A. Langmuir 1996, 12, 2866-2867. (12) Kaneko, K.; Murata, K. Adsorption 1997, 3, 197-208. (13) Benard, P.; Chahine, R. Langmuir 1997, 13, 808-813. (14) Chen, J. H.; Wong, D. S. H.; Tan, C. S.; Subramanian, R.; Lira, C. T.; Orth, M. Ind. Eng. Chem. Res. 1997, 36, 2808-2815. (15) Dobruskin, V. K. Langmuir 1998, 14, 3840-3846. (16) Al-Muhtaseb, S. A.; Ritter, J. A. Ind. Eng. Chem. Res. 1998, 37, 684-696. (17) Neimark, A. V.; Ravikovitch, P. I. Calibration of Adsorption Theories. FOA6 Proceedings. 24-28 May 1998; pp 159-164. (18) Do, D. D.; Wang, K. Langmuir 1998, 14, 7271-7277. (19) Murata, K.; Kaneko, K. Chem. Phys. Lett. 2000, 321, 342-348. (20) Ustinov, E. A.; Do, D. D.; Herbst, A.; Staudt, R.; Harting, P. J. Colloid Interface Sci. 2002, 250, 49-62.
Published on Web 09/01/2009
DOI: 10.1021/la901956g
13461
Article
this kind of model and the classical adsorption theory; therefore, these models did not reveal the nature of the high-pressure adsorption. For example, they did not answer the question of adsorption mechanism at high pressures. Without understanding the mechanism of adsorption at high pressures, miscellaneous materials have been developed and claimed to be efficient for the storage of hydrogen or natural gas, and novel storage materials will continue to appear in the future. Unfortunately, no one material solved the storage problem. Similarly, there is no guidance in the development of adsorbents addressing the separation without understanding the mechanism of high-pressure adsorption. Therefore, the second theoretical problem in highpressure adsorption is to clarify the adsorption mechanism. The third and most essential problem in the theoretical study of highpressure adsorption is, as was indicated by Jaroniec,21 Kaneko,19 and their co-workers, to determine the quantity of absolute adsorption based on the experimental data of surface excess adsorption. Why is it essential? First, the difference between the surface excess and the absolute adsorption becomes larger and larger following the increase of adsorption pressure and causes the deviation of isotherms from that observed at subcritical temperatures; therefore, this difference must be accounted for in order to properly explain the isotherms of high-pressure adsorption. Second, the thermodynamic property of adsorbed phase relies on the quantity of absolute adsorption. Without knowing the value of absolute adsorption, evaluation of the thermodynamic property for the adsorbed phase is absolutely impossible. Finally, high-pressure adsorption will be fully explained with the classical Gibbs definition formula22 if the quantity of absolute adsorption is somehow determined. Keeping these problems in mind, the authors carried out researches on the fundamentals of highpressure adsorption for the past more than ten years. Considering the situation that researchers working on novel materials addressing to hydrogen or natural gas storage may not quite familiar with the recent progress in high-pressure adsorption, the authors think it might make scenes to summarize or generalize our studies aiming to provide readers with a systematically organized and brief description of high-pressure adsorption fundamentals. It is shown in the present paper how the above-mentioned problems were tackled and what consequence this yielded in application studies.
2. Ideology and Methods Used To Tackle Theoretical Problems 2.1. Determination of Absolute Adsorption Based on Experimental Excess Isotherms. Experiments show that the difference between the excess and the absolute adsorption becomes remarkable only at relatively high pressures. In fact, this difference cannot be identified for quite a large pressure range before the maximum of isotherms. Therefore, the experimental data for the range of relatively low pressure can be applied for setting up a model that represents absolute adsorption. To properly pick up the applicable data, experimental isotherms experienced twice transformations. On the basis of the potential theory of adsorption, isotherms for different temperatures can be merged into a general isotherm23 as shown in Figure 1, and this (21) Salem, M. M. K.; Braeuer, P.; Szombathely, M. V.; Heuchel, M.; Harting, P.; Quitzsch, K.; Jaroniec, M. Langmuir 1998, 14, 3376-3389. (22) Everett, D. H. Manual of symbols and terminology for physicochemical quantities and units, Appendix II. Part I; Butterworth: London, 1971. (23) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York, 1984. (24) Zhou, Y. P.; Zhou, L.; Bai, S. P.; Yang, B. Ads. Sci. Technol. 2001, 19, 681-690.
13462 DOI: 10.1021/la901956g
Zhou and Zhou
Figure 1. General isotherm of N2 on activated carbon for 138-298 K.24
rule applies for all adsorptive and adsorbents.24 All points outside the general isotherm are skipped in subsequent processing because they do not conform to the theory for absolute adsorption and must correspond to the condition of overhigh pressures. The general isotherm can be modeled with a Langmuir type equation,24 which was then expanded into Taylor series.25 Higher order terms of the series were skipped in further processing because they may correspond to higher local concentration of adsorbates. A linear model of isotherms shown in Figure 2 was thus obtained. The linear model generates the quantity of absolute adsorption in corresponding to each experimental excess quantity. The volume of adsorbed phase is then evaluated based on the Gibbs definition formula of adsorption:22 va ¼
ns -n Fg
ð1Þ
Here va is the volume of adsorbed phase, Fg is the gas phase density, and ns and n are the absolute and excess quantity of adsorption, respectively. According to eq 1, the adsorption system is divided into two distinct phases, gas phase and adsorbed phase. The two-phase model is sometimes criticized and claimed not true for “supercritical fluid” because of the strong penetration and dissolution property of the fluid as is used for supercritical extraction. The fluid behavior for the critical zone is stochastic; therefore, the two-phase model might be challenged. However, the interaction with solid surface will definitely exerts an effect on fluid behavior, though to what extent is still known little. Nonetheless, the two-phase model of adsorption has never accounted a serious challenge outside the critical zone. The absolute adsorption is determined based on the experimental values of excess adsorption; therefore, measurement of the latter must be reliable. Although there are different measurement methods for high-pressure adsorption,27 gravimetric and volumetric method are the most widely applied in practice. The gravimetric method is suitable for measuring the adsorption on little quantity of sample with satisfactory precision. The relative (25) Zhou, L.; Zhou, Y. P. A. Chin. J. Chem. Eng. 2001, 9, 110-115. (26) Zhou, Y. P.; Bai, S. P.; Zhou, L.; Yang, B. Chin. J. Chem. 2001, 19, 943-948. (27) Zhou, L. Adsorption Isotherms for the Supercritical Region. In Adsorption: Theory, Modeling & Analysis; Toth, J., Ed.; Marcel Dekker: New York, 2002. Chapter 4, Sec. III A.
Langmuir 2009, 25(23), 13461–13466
Zhou and Zhou
Article
Figure 2. Linear isotherms of N2 on activated carbon for 138-
298 K.26
errors do not accumulate in the procedure of increasing adsorption pressure. However, it usually takes a long time to reach a stable reading because the microbalance is very sensitive to any temperature gradient possibly existed inside the instrument space, which may subject to the influence of room temperature fluctuation. The pressure range of gravimetric measurement is usually less than 1 MPa. It enlarged to 10-15 MPa recently with excessive cost. The pressure range of volumetric measurement is high, usually higher than 15 MPa with low cost. It may reach satisfactorily high precision, though the relative errors of measurement accumulate in the procedure of increasing adsorption pressure. The necessary condition for a reliable measurement includes the following: large quantity of sample; temperature and pressure are precisely controlled; an appropriate equation of state for testing gas if the adsorption pressure is higher than 0.5 MPa. 2.2. Isotherm Modeling. After having been determined the absolute adsorption, all quantities in eq 1 are of known value or have been calculated with an equation of state; therefore, the Gibbs definition formula can be applied for modeling the experimental isotherms of high-pressure adsorption: n ¼ ns -Va Fg
ð1aÞ
The isotherms of high-pressure adsorption show type-I features before the isotherm maximum, and all conventional isotherm models were initially derived for the absolute adsorption; therefore, any available type-I isotherm equation is legal to be applied in eq 1a for the quantity of ns. However, those of three parameters better fit the experimental data. Although the absolute adsorption was determined based on part of experimental data, the model obtained fits the experimental data very well for the whole range of experiments as shown in Figure 3 for the adsorption of CO2 on activated carbon.28 In addition, all parameters of the model have reasonable physical meaning.29 Therefore, high pressure adsorption does not claim new theory, and the adsorption isotherm does not represent a new type of isotherms. 2.3. Adsorption Mechanism at High Pressures. The type or shape of adsorption isotherms is determined by intrinsic (28) Zhou, L.; Bai, S. P.; Su, W.; Yang, J.; Zhou, Y. P. Langmuir 2003, 19, 2683-2690. (29) Zhou, L.; Zhang, J. S.; Zhou, Y. P. A. Langmuir 2001, 17, 5503-5507.
Langmuir 2009, 25(23), 13461–13466
Figure 3. Adsorption isotherms of CO2 on activated carbon for
the supercritical range.28 Key: dots, experimental data; curves, model.
adsorption mechanism. Therefore, the unique type of isotherms at high pressures implies unique mechanism of adsorption. The mechanism is monolayer surface coverage, which is supported by multiple proofs: 2.3.1. BET Theory Corollary. The BET theory30 claims that the first molecular layer that fixes on solid surface is due to the interaction between gas and solid. More gas molecules may be adsorbed above the first adsorbed layer due to the interaction among adsorbate molecules and form the second and subsequent layers. The interaction energy for the first layer and the second and subsequent layers is thus different. This difference is reflected in the heat of adsorption for different layers. The experiment of nitrogen adsorption on carbon black31 showed that the heat of adsorption for the first layer is 11 to 12 kJ/mol (0.11 to 0.12 eV), and it dropped to 5.56 kJ/mol (0.058 eV) in the second and subsequent layers. The latter is quite same as the latent heat of condensation. Obviously, the second and subsequent layers cannot exist at above-critical temperatures due to the incondensability of gases. 2.3.2. Evidence of Isotherm Modeling. The adsorption isotherms of methane on activated carbon shown in Figure 4 were modeled by the aforementioned strategy.32 It was assumed in the model that just one layer of methane molecules was adsorbed. Apparently, the model fits experimental data very well. In addition, the intermolecular distance was calculated from a model parameter for the adsorbed phase and from gas density for the gas phase as shown in Figure 5. The intermolecular distance in two phases varies linearly with temperature and intersects at a distance of 0.34 nm, which is same as the dynamic diameter of a carbon atom.33 This result means the mechanism of monolayer surface coverage is quite reasonable for the adsorption. 2.3.3. Evidence of in Situ FTIR Measurements. To study the transformation of adsorption mechanism on crossing the (30) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-319. (31) Beebe, B. A.; Biscoe, J.; Smith, W. R.; Wendell, C. B. J. Am. Chem. Soc. 1947, 69, 95-101. (32) Zhou, L.; Zhou, Y. P.; Li, M.; Chen, P.; Wang, Y. Langmuir 2000, 16, 5955-5959. (33) Aukett, P. N.; Quirke, N.; Riddiford, S.; Tennison, S. R. Carbon 1992, 30, 913-920.
DOI: 10.1021/la901956g
13463
Article
Zhou and Zhou
Figure 4. Adsorption isotherms of methane on activated carbon. Dots: experimental. Curves: model predicted.32.
Figure 5. Calculated intermolecular distances: (O) in gas phase; (b) in adsorbed phase.32.
critical temperature, CO2 isotherms on activated carbon were collected at different temperatures, and the average number of molecule layers adsorbed was calculated.28 While the number is 1.20 at 307 K, it reduces to 1.0 or less at 323 K and higher temperatures. The critical temperature of CO2 is 304 K, therefore, 307 K is still in the critical zone, and multilayer adsorption is possible to occur at some cites. However, multilayer adsorption is never observed as temperature further increases. This result was later proven by the in situ FTIR spectroscopy study for the nearcritical CO2 in mesoporous silica34 aimed to tell whether multilayer or monolayer adsorption really occurred on the surface of adsorbent. The transition of adsorption mechanism from multilayer to monolayer on crossing the critical zone was also shown by the adsorption of nitrogen and methane on mesoporous silica gel.35 The monolayer surface coverage mechanism of high-pressure adsorption may confront disputation based on arguments that follow. First, the gas molecules confined in a space of nano dimension may receive ultra ordinary action exerted by surrounding walls and, therefore, may assume liquid or similar state. However, there is not any support to the argument either from experiments or molecular simulation. According to a molecular dynamics simulation,36 a hydrogen atom with dynamic moment 20 eV was transplanted through the wall into a tube of diameter 0.683 nm composed of 150 carbon atoms. It was found that hydrogen atoms were recombined to form molecules and arranged concentrically inside the tube. Pressure inside the tube reached to 350 thousand atmospheres when 90 hydrogen atoms were implanted (corresponding to a storage density 5 wt %). No condensation was shown even at such high pressure. Second, following the increase of pressure, the density of incondensable gas keeps increasing and molecules tend to settle down orderly above solid surface at very high pressures and, thus, the monolayer mechanism no longer functions. However, there is also an upper limit for high-pressure adsorption, though the saturation
pressure does not exist.35 Adsorption is a phenomenon caused by internal forces among molecules/atoms. The orderly settling down of gas molecules is a result caused by external forces and beyond the scope of adsorption already.
(34) Schneider, M. S.; Grunwaldt, J. D.; Baiker, A. Langmuir 2004, 20, 2890-2899. (35) Zhou, L.; Zhou, Y. P.; Bai, S. P.; Yang, B. J. Colloid Interface Sci. 2002, 253, 9-15. (36) Ma, Y.; Xia, Y.; Zhao, M.; Wang, R.; Mei, L. Phys. Rev. B 2001, 63, 115422.
13464 DOI: 10.1021/la901956g
3. Progress in Application Studies Promoted by Theoretical Progress Once barriers in understanding high-pressure adsorption were overtaken, new progress in application studies appeared. For example, we have the following points. 3.1. Improvement in Modeling Multicomponent Adsorption. Multicomponent adsorption equilibrium is the basis of adsorption process simulation. Unlike the equilibrium between fluid phases, the model for gas/solid phase equilibrium cannot be derived out of thermodynamics laws or rules. It relies on the models for single component adsorption and an adjustment between them. No matter if a model is theoretically or empirically deduced, it must pass the examination of multicomponent adsorption experiments. The experiment of multicomponent adsorption is difficult to carry on, more components, more difficult. Therefore, only few experimental data are available for two-component mixtures, and less for three and more component mixtures in literature. To properly adjust the relation between single component adsorption, the difference in adsorption mechanisms between components must be considered. The adsorption temperature may be higher than the critical temperature of some components, and lower than the critical temperature of others. As such, monolayer or multilayer adsorption must be identified for each component as shown in Figure 6.37 After identifying the adsorption mechanism of components and taking the adsorption mechanism into the prediction model, remarkable improvement was achieved in predicting multicomponent adsorption equilibrium. As shown in Figure 7, parts a and b, for the fourcomponent mixture composed of H2, CH4, CO2 and N2, the relative error in predicting component adsorption as well as the composition of adsorbed phase is remarkably less than that for the other prediction models.37 (37) Wu, J. Q.; Zhou, L.; Sun, Y.; Su, W.; Zhou, Y. P. AIChE J., 2007, 53, 1178-1191.
Langmuir 2009, 25(23), 13461–13466
Zhou and Zhou
Article
Figure 6. Classification of adsorbed molecules.37.
Figure 7. Comparison of models in predicting (a) component adsorption and (b) the composition of the adsorbed phase.37 Note: FHVSM, Flory-Huggins vacancy solution model; LRC, loading ratio correlation; MREL, multiregion extended Langmuir; MPSD,micropore size distribution; MISC, model including supercritical components; EL, extended Langmuir; IAST, ideal adsorbed solution theory.
3.2. Guidance Research on Hydrogen Storage. Miscellaneous materials, from activated carbon, carbon nanotubes to MOF’s (metal organic frameworks) and COF’s (covalent organic frameworks), have been proposed for hydrogen storage so far, and more novel materials are definitely to appear in the future for the same purpose. However, the hydrogen uptake capacity of any material is limited by the specific surface area according to the adsorption mechanism of monolayer surface coverage provided the storage temperature is higher than 33 K. Other feature or property of the material will not exercise an essential effect on the capacity as is experimentally shown that the uptake capacity of hydrogen by different materials is linearly correlated with the specific surface area as shown in Figure 8 by the author38 and other researchers.39,40 It is, therefore, concluded that there is not a hydrogen carrier that satisfies commercial requirement on storage density as long as the storage principle is adsorption. 3.3. Guidance Research on Natural Gas Storage. Similar to hydrogen storage, miscellaneous materials were proposed for
natural gas storage, and the natural gas stored this way was recognized as ANG (adsorbed natural gas). According to the mechanism of monolayer surface coverage, the storage capacity of methane or natural gas is also limited by the specific surface area of materials. Adding other features to adsorbents cannot change the linear relationship between the adsorbed amount and the specific surface area as shown in Figure 9.41 Therefore, if someone claimed a novel material of extremely large storage capacity for methane,42 it must be wrong somewhere.41 The ANG technology also has other technical drawbacks and does not seem to possess commercial potential. New storage technology based on a principle other than adsorption should be developed for the storage of natural gas. The wet storage method is thus proposed.43 Instead of dry carbon, wet carbon was used as the natural gas carrier. However, the storage mechanism switched from adsorption to hydrate formation in the pore and void spaces. All technical drawbacks of the ANG were overcome by the wet storage method, and the storage capacity can be as high as more
(38) Chu, X. Z.; Zhou, Y. P.; Su, W.; Sun, Y.; Zhou, L. J. Phys. Chem. B 2006, 110, 22596-22600. (39) Str€obel, R.; J€orisen, L.; Schliermann, T.; Trapp, V.; Sch€utz, W.; Bohmhammel, K.; Wolf, G. G. J. Power. Source 1999, 84, 221-224. (40) Nijkamp, M. G.; Raaymakers, J. E. M. J.; van Dillen, A. J.; de Jong, K. P. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 619-623.
(41) Sun, Y.; Liu, C. M.; Su, W.; Zhou, Y. P.; Zhou, L. Adsorption 2009, 15, 133-137. (42) Ma, S. Q.; Sun, D. F.; Simmons, J. M.; Collier, C. D.; Yuan, D. Q.; Zhou, H. C. J. Am. Chem. Soc. 2008, 130, 1012-1016. (43) Zhou, L.; Sun, Y.; Zhou, Y. P. AIChE J. 2002, 48, 2412-2416.
Langmuir 2009, 25(23), 13461–13466
DOI: 10.1021/la901956g
13465
Article
Figure 8. Dependence of adsorption amount on specific surface area. Light marks: H2. Dark marks: D2.38
Zhou and Zhou
Figure 10. Linear relationship between separation coefficient and specific surface area (adsorbents include different activated carbons, zeolites, and silica gels).46
enrichment of methane from coalbed gas is very important, but is also a big challenge since the pair of key components, CH4/N2, is difficult to separate by any means. PSA (pressure swing adsorption) is a low cost technology, especially for enriching a low-concentration component. However, the adsorptive separation coefficient is about 2-6 for most available adsorbents, which is not large enough to lower the separation cost. How to develop an adsorbent that possesses a large separation coefficient for the key components CH4/N2 was considered a big challenge for the adsorption community.45 Both CH4 and N2 are supercritical gases at ambient temperature; therefore, their adsorption must follow the mechanism of monolayer surface coverage, and the separation coefficient was enlarged by increasing the specific surface area of adsorbent as shown in Figure 10.46
4. Concluding Remark Figure 9. Methane adsorption on different materials at 3.5 MPa and 298 K.41 Key: 1, MWCNT; 2, zeolite 13X; 3, silica gel; 4, MCM41; 5, H103 (adsorption resin); 6, ACF1 (activated carbon fiber 1); 7, CMK-3 (carbon mesoporous molecular sieve); 8, ACF2 (activated carbon fiber 2); 9, ML-L (hyper-cross-linked polymer network); 10, AC-BY0 (an activated carbon); 11, PCN-14 (a MOF material); 12, AC-LM1 (an activated carbon); 13, AC-BY1 (an activated carbon); 14, AC-paper (activated carbon paper); 15, ACLM3 (an activated carbon); 16, AC-BY2 (an activated carbon).
than 40 wt % (compared to less than 20 wt % in ANG) by tuning up pore sizes and moisture content.44 3.4. Guidance Research on Coalbed Methane Enrichment. Coalbed methane is one of the sources of greenhouse gas and a cause of coal mine disasters; however, it is also an abundant source of clean fuel. To facilitate transportation and utilization, the content of methane should be higher than 80%. Therefore, (44) Liu, X. W.; Zhou, L.; Li, J. W.; Sun, Y.; Su, W.; Zhou, Y. P. Carbon 2006, 44, 1386-1392.
13466 DOI: 10.1021/la901956g
Application studies came up with problems calling for solutions from the theoretical level. How to understand the fundamentals of high-pressure adsorption is a serious challenge; however, any progress in fundamental research must promote the progress of application researches, which is, in turn, a criterion to judge whether the theoretical progress is valid. There are still a lot of practical requirements for high-pressure adsorption; therefore, more challenges and opportunities for success are on the way. Acknowledgment. Financial support of the National Natural Science Foundation of China for the fundamental research on high-pressure adsorption (Grant Nos. 29936100 and 20336020) is sincerely acknowledged. The authors are grateful to all graduate students consecutively working in our laboratory for their diligent and excellent jobs. (45) Ruthven, D. M. Ind. Eng. Chem. Res. 2000, 39, 2127-2131. (46) Zhou, L.; Guo, W. C.; Zhou, Y. P. A. Chin. J. Chem. Eng. 2002, 10, 558-561.
Langmuir 2009, 25(23), 13461–13466
