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Aqueous and Gaseous Adsorption from Montmorillonite-Carbon Composites and from Derived Carbons A. Bakandritsos,† E. Kouvelos,‡ Th. Steriotis,‡ and D. Petridis*,† Institute of Materials Science and Institute of Physical Chemistry, N.C.S.R. Demokritos, Agia Paraskevi 153 10, Athens, Greece Received October 11, 2004. In Final Form: December 14, 2004 Clay-carbon composites and the carbons derived from demineralization of the clay template were examined for their aqueous adsorption properties (2,4,6-trichlorophenol and methylene blue) and for their gas adsorption/separation abilities regarding CO2, CH4, and N2 gases. The sorption results are discussed in relation with their structural properties (surface area, pore width and volume, and surface chemistry). It was found that the properties of the adsorbents depend highly on the synthetic route, for instance, on the use of clay or H2SO4 as structure mediating and activating agents, respectively. Particularly, the simultaneous use of clay and H2SO4 leads to a synergistic action, which imparts to the final solids the highest sorption capacity and the best potential for separation of CO2 from gaseous mixtures of CH4 and N2.
Introduction The search for effective and of low cost absorbents to eliminate present and future harmful developments, to restore polluted environments, and to treat hazardous wastes is a scientific and technological issue of paramount importance. Activated carbons occupy a unique position in the hierarchy of absorbent materials for cleaning air, gases, water, and wastes from chemical operations. They possess high adsorption capacities, arising mainly from their microporous but also mesoporous and even macroporous nature. Analogous porous inorganic materials have also been investigated and developed for adsorption and separation processes. Examples include natural and synthetic zeolites,1-3 clay minerals4,5 and their organic derivatives,1,6 and natural microporous oxides (γ-alumina, ferrites, goethite, manganese oxides).1,7 Also, carbon molecular sieves are materials essential in technological processes of high value, such as liquid adsorption and separation of gaseous mixtures.8,9 For instance, CO2 gas separation is important in the upgrade of natural gas and decarbonization of gasified fuels, while its capture and * To whom correspondence should be addressed. Tel.: 003 210 6503343. Fax: 003 210 6519430. E-mail: dpetrid@ ims.demokritos.gr. † Institute of Materials Science, N.C.S.R. Demokritos. ‡ Institute of Physical Chemistry, N.C.S.R. Demokritos. (1) For a general description of the sorptive and other properties of natural microporous materials (zeolites, clays, micas, Fe/Mn-oxides/ hydroxides/oxyhydroxydes) see Natural Microporous Materials in Environmental Technology; Misaelidis, P., Maca´sˇek, F., Pinnavaia, T. J., Collela, C., Eds.; NATO Science Series, E; Kluwer: Dordrecht, 1999; Vol. 362. (2) Ming, D. W., Mumpton, F. A., Eds. Natural Zeolites’93: Occurrence, Properties, Use; International Committee on Natural Zeolites, ICNZ Publications: Brockport, NY, 1995. (3) Milton, R. M. U.S. Patents 2,882,243 and 2,882,244, 1959. (4) Zhu, H. Y.; Vansant, E. F.; Lu, G. Q. J. Colloid Interface Sci. 1999, 210, 352. (5) Bandosz, T. J.; Jagiello, J.; Putyera, K.; Schwarz, J. A. Chem. Mater. 1996, 8 (8), 2023. (6) Xu, S.; Sheng, G.; Boyd, S. A. Adv. Agron. 1997, 59, 25. (7) Galias, G. P., Matis, K. A., Eds. Mineral processing and the Environment; Kluwer Academic Publishers: Dordecht, 1998. (8) Pinavvaia, T. J., Thorpe, M. F., Eds. Access in Nanoporous Materials; Plenum Press: New York, 1995; p 39. (9) Prasetyo, I.; Do, D. D. Carbon 1999, 37, 1909.
storage are very important processes in reducing the greenhouse emissions. A major factor controlling the sorptive and sieving properties of adsorbents is the selective size exclusion of molecules due to pore constriction or/and kinetic parameters that define the diffusion of molecules throughout the adsorbent volume. Equally important is the role of chemical groups present on the adsorbent surfaces because such groups can affect diffusion and sorption through van der Waals, hydrogen bonding, or other polar interactions.10 For these reasons, the exploitation of existing or the synthesis of new adsorbents with desirable pore structure and surface chemistry is a matter of great significance. Natural or synthetic inorganic solids with a rigid threedimensional pore framework, such as zeolites11 and mesoporous crystalline materials,12 are good candidates in this direction. In the area of synthesis of new adsorbents the template technique is a particularly useful route in which an inorganic substrate is combined with carbonaceous precursors to produce, after pyrolysis, porous inorganic-carbon composites. These composites yield bulk carbons after dissolution of the inorganic matrix. For instance, zeolites,13-15 MCM-48,16 SBA-15,17 and SiO2 spheres18 have been applied successfully as structuremediating agents for the formation of ordered carbons. Recently, we have described the synthesis and characterization of clay-carbon composites and of the pure (10) Radovic, L. R., Ed. Chemistry and Physics of Carbon; Marcel Dekker: New York, 2000; Vol. 27, pp 227-382. (11) Takahashi, A.; Yang, R. T.; Manson, C. L.; Chinn, D. Langmuir 2001, 17 (26), 8405. (12) Xu, X. C.; Song, C. S.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Microporous Mesoporous Mater. 2003, 62 (1-2), 29. (13) Meyers, C. J.; Shah, S. D.; Patel, S. C.; Sneeringer, A. M.; Bessel, C. A.; Dollahon, N. R.; Loising, R. A.; Takeuchi, E. S. J. Phys. Chem. B 2001, 105 (11), 2143. (14) Barata-Rodrigues, P. M.; Mays, T. J.; Moggridge, G. D. Carbon 2003, 41, 2231. (15) Kyotani, T.; Nagai, T.; Inoue, S.; Tomita, A. Chem. Mater. 1997, 9, 609. (16) Ryo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103 (37), 7743. (17) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122 (43), 10712. (18) Lei, Z.; Zhang, Y.; Wang, H.; Ke, Y.; Li, J.; Li, F.; Xing, J. J. Mater. Chem. 2001, 11, 1975.
10.1021/la047495g CCC: $30.25 © 2005 American Chemical Society Published on Web 02/08/2005
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Table 1. Abbreviations and Identities of the Studied Solids composites after pyrolysis at 873 K Z-Na+/C 32% in carbon Z-H+(w)/C 29% in carbon Z-H+(uw)/C 26% in carbon Sug-H+/C
derived carbons after clay removal
montmorillonite/carbon composite Carbon from the composite without without addition of H2SO4 addition of H2SO4 montmorillonite/carbon composite wCarbon from the composite with addition of H2SO4 in the presence of H2SO4 and washing and washing prior to pyrolysis montmorillonite/carbon composite uwCarbon from the composite with addition of H2SO4 in the presence of H2SO4 without washing and without washing prior to pyrolysis carbon obtained after sucrose was impregnated with H2SO4 and carbonized at 873 K without previous washing
carbons obtained after removal of the mineral.19 Several products were prepared according to whether the synthesis was performed in the presence of the clay and with or without H2SO4 activation. The present work reports the aqueous and gaseous adsorption properties of the clay/ active carbon composites and of the resulting carbons in connection with their structural characteristics. It is shown how the selected synthetic procedure can tailor the final structural characteristics with the aim to improve the properties of the solids to be used in applications such as removal of organic pollutants from aqueous media and natural gas upgrade. Experimental Section The synthesis and characterization of the aforementioned products have been presented in an earlier publication.19 Briefly, commercial table sugar intercalated in montmorillonite, in the presence or absence of H2SO4, was pyrolyzed at 873 K to yield the clay-carbon composites. The H2SO4 was either washed out or not from the solids before pyrolysis at 873 K. The pure carbon phases were isolated after demineralization of the clay matrix. The presence of H2SO4 had a synergistic effect on a “short-range” template effect of the inorganic matrix, imparting high surface area and mesoporosity to the products. In Table 1 the abbreviations and identity of the studied composites and carbons are explained. 2,4,6-Trichlorophenol (2,4,6-TCP), a toxic organic pollutant, and methylene blue (MB), a cationic dye, were selected as representative molecules for sorption applications from aqueous media. Sorption isotherms of 2,4,6-TCP and MB were carried out using the batch equilibrium technique.20 For each isotherm 25 mg of the adsorbent was weighted into a 50-mL Erlenmeyer flask, followed by addition of aqueous solutions of 2,4,6-TCP or MB in known concentrations between 3 and 125 ppm. The flasks were stoppered and shaken for 24 h at 298 K to ensure adsorption equilibrium. Preliminary experiments showed that equilibrium was achieved within 24 h. The contents of each flask were filtered, and the concentration of the supernatant was determined from the absorption at λ ) 508 nm for 2,4,6-TCP and at λ ) 290 nm for MB in a UV-vis spectrophotometer Shimadzu UV2100. Solutions of 2,4,6-TCP were measured following known procedures,21 while MB solutions could be measured directly. In the experimental adsorption isotherms, the quantity in milligrams of the organic molecules adsorbed per gram of the solid is presented against the residual equilibrium quantity (mgeq) of trichlorophenol or MB. The adsorbed mg is estimated by subtracting the residual or equilibrium quantity of the organic molecules from that initially added. The most common twoparameter equations are the Freundlich and Langmuir isotherms.22 The Freundlich equation can be applied to sorption on heterogeneous surfaces as well as multilayer sorption, while Langmuir refers to monolayer coverage. The Langmuir model is probably the best known and most widely applied sorption isotherm producing good agreement with a variety of experi(19) Bakandritos, A.; Steriotis, Th.; Petridis, D. Chem. Mater. 2004, 16, 1551. (20) Furuya, E. G.; Chang, H. T.; Miur, Y.; Noll, K. E. Sep. Purif. Technol. 1997, 11, 69. (21) Snell, F. D.; Snell, C. T. Colorimetric Methods of Analysis; D. Van Nostrand Company, Inc.: New York, 1961; Vol IIIA. (22) (a) Freundlich, H. M. F. J. Phys. Chem. 1906, 57, 385. (b) Langmuir, I. J. Am. Chem. Soc. 1916, 40, 1361.
mental data. The Langmuir model was also employed in this study to fit the experimental isotherms, which were in the following reduced to their linear form. The squared values of the coefficients (r2) for all the linear regressions were determined to be higher than 0.99. The maximum adsorption capacities were deduced from the linear equation. Fourier transform infrared (FT-IR) spectra for the surface chemistry characterization of the adsorbents were collected on a Bruker infrared spectrometer in the form of KBr pellets. Spectra were collected with a resolution of 2 cm-1 and with 50 scans. The pellets were prepared by mixing 100 mg of KBr with 5 mg of solid in the case of composites and 2 mg in the case of pure carbons. N2, CO2, and CH4 adsorption-desorption isotherms of the samples were measured gravimetrically in a pressure range 0-2 MPa, at 253 K, by means of an Intelligent gravimetric analyzer (IGA, Hiden Isochema). The samples were outgassed at 523 K overnight, under ultrahigh vacuum (10-5 Pa). The experimental results have been carefully corrected for buoyancy effects taking into account the compressibility of gases. The mass and density of hang-down systems (Au chains), sample and counterbalance holders (porous stainless steel and glass, respectively), and counterbalance (stainless steel rods) have been precalculated, while the skeletal density of the samples was measured by helium pycnometry. No buoyancy corrections for the adsorbed phase were carried out; thus, the results refer to excess isotherms. N2 adsorption isotherms at 77 K have been performed volumetrically on an Autosorb-1 gas analyzer, Micropore version (Quantachrome instruments). The total surface areas, St, were calculated from the Brunauer-Emmett-Teller equation, while nonmicropore (i.e., mesopore + external) surface areas, Smes+ext, and micropore volumes, Vmic, were determined by the slope and intercept of the linear initial part of Rs plots,23 produced after using TK-900 SiO2 (Degussa) as the nonporous reference material. The total pore volume, Vt, was assumed to be the liquid volume of nitrogen at relative pressure, p/p0 ) 0.99. The micropore surface area, Smic, and the nonmicropore (i.e., mesopore + macropore) volume, Vmes+mac, were determined as Smic ) St - Smes+ext and Vmes+mac ) Vt - Vmic. N2 adsorption-desorption isotherms at 77 K are provided in the Supporting Information.
Results and Discussion 1. Aqueous Adsorption. In sorption studies the shape of an isotherm provides a first experimental evidence for the qualitative analysis of the nature of the adsorption. According to the ordinary classification of isotherm shapes,24 two types of isotherms were observed in the present materials: Langmuir (L) type and high affinity (HA) type. The clay-carbon composites resemble the L type model, Figure 1a. The HA type was observed for the derived carbons, that is, for Carbon, wCarbon, and uwCarbon products, Figure 1b. The HA type isotherms, characterized by a very steep initial rise followed by a plateau, demonstrate the strong sorption of 2,4,6-TCP by the respective adsorbents. The experimental isotherms were fitted to the linear Langmuir equation: Ce/Q ) 1/(qmb) + (1/qm)Ce, where Q is the uptake capacity in mg/g, Ce is the residual equilibrium concentration in mg/L of the (23) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (24) Lyklema, J. Fundamentals of interface and Colloid Science, Solid-liquid interfaces; Academic Press: New York, 1995; Vol. II.
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Figure 1. Sorption isotherms expressed in milligrams of sorbed 2,4,6-TCP and MB per gram of the adsorbents versus the residual/ equilibrium quantity of the adsorbates.
adsorbate, b is the Langmuir constant in L/mg, and qm is the maximum uptake capacity of the adsorbent in mg/g. To properly fit the HA isotherms to the linear Langmuir equation, the data at the initial steep rise were excluded from the calculation. The observed differences in uptake capacities can be explained by considering the factors that affect adsorption in porous materials. As has been described before, the adsorption properties of porous materials are related to several factors, including (i) surface area and pore volume of the adsorbents; (ii) the accessibility of the adsorbates to the inner surfaces of pores, which clearly depends on the size of the pores and of the entering molecules;25,28 (iii) the surface chemistry of the adsorbent, for instance, the presence of heteroatoms and functional groups; (iv) the hydrophobic or hydrophilic properties of the adsorbates and adsorbents, for instance, in aqueous systems the adsorption of hydrophobic molecules by hydrophobic adsorbents is mainly governed by their dislike of water and their affinity for the adsorbent’s surface; and (v) in the case of active carbons, the mineral content may
introduce deleterious effects on the adsorption25 by blocking the porosity of the adsorbent or from preferential adsorption of hydrophilic molecules (e.g., water) instead of an organic hydrophobic compound. In the consideration of these effects, the adsorption of 2,4,6-TCP and MB will be discussed separately because the two molecules have different dimensions and exhibit different hydrophobicities. 2,4,6-TCP is a hydrophobic molecule, while MB is hydrophilic with a positive charge. It is noted that the charge of MB is not very important in the present experiments because montmorillonite has lost its cation exchange capacity in samples Z-H+(w)/C and Z-H+(uw)/ C, while it retains only part of its exchange properties in sample Z-Na/C. Adsorption of 2,4,6-TCP. An immediate conclusion from Figure 2 is the much higher adsorption capacities in “mg/ g” of the derived carbons (5 times higher in the case of uwCarbon) than the respective composites. Removal of the clay affords solids with highly increased pore volume and surface area because of the developed microporosity (25) Moreno-Casstilla, C. Carbon 2004, 42, 83.
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Figure 2. Diagram displaying the maximum uptake capacities of composites and derived carbons in milligrams per gram. Table 2. Total (St), Micropore (Smic), and Mesopore + External (Smes+ext) Specific Surface Areas; Total (Vt) Micropore (Vmic) and Mesopore + Macropore (Vmes+mac) Pore Volumes; and Uptake Capacities Expressed in mg/m2 of the Solids [) Uptake (mg/g)/St] solid
St (m2/g)
Smic (m2/g)
Smes+ext (m2/g)
Vt (cm3/g)
Vmic (cm3/g)
Vmes+mac (cm3/g)
uptake mg/m2 (TCP)
uptake mg/m2 (MB)
Z-Na+ Z-Na+/C Z-H+(w)/C Z-H+(uw)/C Carbon wCarbon uwCarbon Sug-H+/C
36 59 57 521 680 681 1290 621
3 8 7 367 220 227 487 525
33 51 50 154 460 453 804 96
0.11 0.13 0.12 0.41 0.94 0.97 1.48 0.29
0.00 0.00 0.01 0.15 0.10 0.10 0.22 0.21
0.10 0.13 0.12 0.26 0.85 0.87 1.26 0.08
0.91 0.96 0.11 0.23 0.31 0.2 0.02
0.95 0.91 0.26 0.25 0.27 0.21 0
and mesoporosity, Table 2, which are, generally, responsible for the highly enhanced capacities. The very important role of the clay template is underlined if results are compared to the capacity of the pure carbon resulted from bulk sugar after H2SO4 treatment and pyrolysis (SugH+/C). This carbon exhibits very poor sorption properties, that is, 12 mg/g, in contrast to the 156-260 mg/g capacities of the templated carbons, Figure 1e. It is emphasized that the only difference between the templated carbons and Sug-H+/C is that the former were generated inside the lamellar space of montmorillonite. The high discrepancies in the sorption capacities could be justified from the pore volume and surface area distribution of the three templated carbons, Carbon, wCarbon, and uwCarbon and of the Sug-H+/C, Table 2. The last adsorbent possesses a highly microporous framework (85% of the total surface area), whereas the templated carbons display high mesoporosity (>60% of the total surface area). Taking into account that trichlorophenol is a molecule with a mean diameter of 10 Å,26 its diffusion through micropores (pore size < 20 Å) and its adsorption by a highly microporous solid will be limited, because the physical absorption of a molecule requires pores of approximately 1.7 times bigger that its mean size.27,28 Therefore, the template role of the clay is crucial for shaping the pore structure of the derived carbons into a mesoporous network, which enhances the sorption of 2,4,6-TCP and, as shown later, that of MB too. Another matter to be discussed is that Smes+ext for SugH+/C is higher that that of the composites but hardly adsorbs any amount of 2,4,6-TCP. This is not in contrast (26) Hu, Z.; Srinivasan, M. P.; Ni, Y. Carbon 2001, 39, 877. (27) Kasaoka, S.; Sakata, Y.; Tanaka, E.; Naitoh, R. Int. Chem. Eng. 1989, 29 (4), 734. (28) Pelekani, C.; Snoeyink, V. L. Carbon 2000, 38, 1423.
to the previous conclusions because for Sug-H+/C 85% of St corresponds to the micropore surface. Therefore, the mesopore surface is not easily accessible or is not accessible at all by the organic molecules because their diffusion throughout the sorbent’s volume is restricted by the presence of micropores. Interpretation of sorption data regarding composites requires further discussion because they display some inconsistencies. Specifically, in “mg/g” terms, all the composites reveal similar uptakes although one would expect to observe increased capacity for Z-H+(uw)/C because this sample possesses a higher mesopore surface area. Consequently, the combination of constant mass capacity (mg/g) with the dramatic increase of surface area leads to a very low estimation of “surface capacity” (0.11 mg/m2) for Z-H+(uw)/C in contrast to the much higher ∼1 mg/m2 for Z-Na+/C and Z-H+(w)/C. Because the mineral content of all composites is almost 70 wt %, Table 1, their surface properties are similar and such high discrepancies in sorption per squared meter are not expected. A possible reason for this behavior could be the mineral matter content. As already described, mineral matter can either block the entrance to the pores or adsorb preferentially hydrophilic solvents,25 water in our case, instead of the hydrophobic 2,4,6-TCP. The possibility of blocking the pore entrance is small in the present composites, because in this case, the adsorption behavior of Z-Na+/C, Z-H+(w)/C, and Z-H+(uw)/C for 2,4,6-TCP would parallel their adsorption of MB, which is not observed. Instead, the uptake capacity for MB generally follows the increase of their Smes+ext values. Taking these arguments into consideration, we must conclude that the uptake of 2,4,6-TCP by the composites is greatly influenced by the presence of the prevalent hydrophilic clay partner
Aqueous and Gaseous Adsorption
Figure 3. FT-IR spectra of (a) composite Z-Na+/C, (b) composites Z-H+(w)/C and Z-H+(uw)/C with similar spectra, (c) derived carbon uwCarbon, (d) derived carbon wCarbon, and (e) pyrolyzed sugar Sug-H+/C and derived carbon Carbon with similar spectra.
which is engaged in a competitive adsorption for the hydrophilic solvent and the organophilic 2,4,6-TCP and ends up with the prevailing adsorption of the abundant water molecules by the hydrophilic clay surfaces. In normal air humidity montmorillonite adsorbs H2O in the range of 110 to 180 mg/g as deduced from the H2O adsorptiondesorption isotherm at a relative pressure ∼ 0.6, which corresponds to the air humidity conditions (isotherm provided in the Supporting Information). The extremely high hydrophilicity of clay minerals is also exhibited by their exfoliation when dispersed in water, where the clay platelets become fully hydrated. For reasons of comparison we present literature data for trichlorophenol sorption from a commercial active carbon sample, which indicate a capacity of 155 mg/g,29 significantly lower than that of uwCarbon. Regarding the adsorption of 2,4,6-TCP by the clay-carbon composites with a mean capacity of about 60 mg/g, this should be compared with adsorption capacities from clays and clay derivatives. Increased clay organophilicity has been achieved mainly via modification with organic cations.30-32 For instance, capacities up to 20-45 mg/g were reported for organoclays32 and even smaller values were reported for aluminum pillared clays.33 As expected, pure Na+montmorillonite adsorbs very low amounts of 2,4,6-TCP, almost six times less than the present clay-carbon composites. Lining the montmorillonite surfaces with carbon graphenes increases surface organophilicity and, therefore, enhances significantly their uptake capacities for 2,4,6-TCP. Finally, we must emphasize that the successful combination of the clay platelets with carbon graphenes affords hybrid materials which are very stable in purification processes of complex liquid systems, such (29) Dobbs, R. A.; Cohen, J. M. Carbon Adsorption Isotherms for toxic organics; EPA 600/8-80-023, MERL; Office of Research and Development, U. S. Environmental Protection Agency: Cincinnati, OH. (30) Lawrwnce, M. A. M.; Kukkadapu, R. K.; Boyd, S. A. Appl. Clay Sci. 1998, 13, 13. (31) Mortland, M. M.; Shaobai, S.; Boyd, S. A. Clays Clay Miner. 1986, 34 (5), 581. (32) Dentel, S. K.; Bottero, J. Y.; Khatib, K.; Demougeot, H.; Duguet, J. P.; Anselme, C. Water Res. 1995, 29 (5), 1273. (33) Danis, T. G.; Albanis, T. A.; Petrakis, D. E.; Pomonis, P. J. Water Res. 1998, 32 (2), 295.
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Figure 4. Possible routes for transformation of carboxylic groups into ketonic through Friedel-Crafts type reactions where the protonated form of montmorillonite (clay-H+) acts as the Lewis acid.
as, for instance, in purification of seawaters or industrial wastes, unlike organoclays. Clay-carbon composites, although they display smaller capacities in comparison to active carbon, exhibit certain advantages. During regeneration of activated carbons by thermal desorption or thermal oxidation of the toxicants in air, a substantial fraction of the carbon can be lost with each oxidation cycle.34 On the other hand, clays are widely known for their fire-retardant properties that their composites are endowed with and, therefore, clay-carbon sorbents could display enhanced behavior in regeneration processes. Adsorption of MB. The adsorption of MB by the composites is related to the surface areas and pore volume distribution of the adsorbents in a different way than that found for TCP, as already described in the previous section. This behavior could be attributed to the hydrophilic nature of MB, and as previously noted, when pore closure does not occur, the mineral content inhibits sorption of hydrophobic molecules only and not of MB. Therefore, an increase of Smes+ext results in an increase of sorption capacity of the composites. Because MB is a molecule with dimensions of 17.0 × 7.6 × 3.25 Å,35 its entrance to the inner surface of a solid poses the necessity of a mesoporous network. Once more, the beneficial effect of pore widening after clay removal is nicely exhibited by the response of the two carbons SugH+/C and wCarbon in MB sorption. They are characterized by almost the same total surface area, but wCarbon has much lower Smic and much higher Smes+ext, allowing diffusion and sorption of MB, while Sug-H+/C with high Smic and low Smes+ext cannot adsorb detectable amounts of MB. FT-IR Characterization. FT-IR spectra of the adsorbents are shown in Figure 3. The clay-carbon composites, Figure 3a,b, exhibit the characteristic bands of SisOsSi and Sis OsM (MdAl3+, Mg2+, Fe3+,2+) vibrations of the clay lattice at 1044 and 470 cm-1, whereas carbons derived after dissolution of the mineral, Carbon, wCarbon, and uwCarbon, lack these vibrations, Figure 3c-e. All carbonaceous materials have a common absorption at 1580 cm-1 which is attributed to carbon-carbon bond vibrations in (34) Guymont, F. J. In Activated Carbon Adsorption of Organics from Aqueous Phase; Suffet I. H., McGuire, M. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1984; Vol 2. (35) Thi Hang, P.; Brindley, G. W. Clays Clay Miner. 1970, 18, 203.
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Figure 5. Adsorption-desorption isotherms (253 K) of CO2, CH4, and N2, for Z-H+(uw)/C, uwCarbon, and Sug-H+/C.
the aromatic basal planes.36 An important feature is the presence or absence of the carbonyl peak (CdO) appearing at 1690 cm-1.37 This peak is present in the composites Z-H+(w)/C and Z-H+(uw)/C and in the derived carbons uwCarbon and wCarbon, Figure 3b-d, while it is absent in the composite Z-Na+/C and the derived Carbon, Figure 3a,e. The spectra indicate that the presence of H2SO4 during pyrolysis of the clay-sugar complexes promotes the formation of carbonyl groups. Surprisingly, bulk sugar pyrolyzed under the same conditions, Sug-H+/C, does not form such groups, Figure 3e. Apparently, only the simultaneous presence of montmorillonite and H2SO4 favors carbonyl group formation. It is known that sugars in the presence of H2SO4 dehydrate to furfuralic derivatives that are further condensed and polymerized.38 However, with increasing temperature the CdO bonds are cleaved and oxygen is eliminated from the furfuralic derivatives. This is likely the reason for the lack of carbonyl groups in Sug-H+/C. Only when clay is also present during pyrolysis do carbonyl groups appear in the FT-IR spectrum. One plausible explanation is that montmorillonite partially shields the elimination of heteroatoms during pyrolysis. Furthermore, any formed carboxylic derivatives can be transformed to keto-derivatives, because of the H+-montmorillonite form action as a Lewis acid catalyst in many organic reactions,39 for instance, in Friedel-Crafts type processes.40 Possible routes are shown in Figure 4. The presence of carboxylic or carbonylic groups located at the edges of the graphenes affect greatly the π-π electron interactions between the aromatic rings of the active carbon and of the adsorbate.25,41,42 Previous studies have shown that acidic oxygen groups (i.e., carboxylic groups) reduce the phenol uptake by removing electron density from the aromatic rings of active carbon and, thus, weaken (36) Biniak, S.; Szymaski, G.; Siedlewski, J.; Swiatkowski, A. Carbon 1997, 35 (12), 1799. (37) Nakanishi, K. Infra-Red Absorption Spectroscopy-Practical; Holden-Day: San Francisco, 1964. (38) Kirck, R. E.; Othmer, D. F. Encyclopedia of Chemical Technology, 3d ed.; Wiley: New York, 1960; Vol 13, p 238. (39) Ballantine, J. A.; Purnell, J. H.; Thomas, J. M. Clay Miner. 1983, 18, 347. (40) March, J. Advanced Organic Chemistry; Reactions, Mechanisms and Structure, 3rd ed.; John Willey & Sons: New York, 1984; p 486. (41) Coughlin, R. W.; Ezra, F. S.; Tan, R. N. J. Colloid Interface Sci 1968, 28 (3/4), 386. (42) Mattson, J. S.; Mark, H. B., Jr.; Malbin, M. D.; Webber, W. J., Jr.; Crittenden, J. C. J. Colloid Interface Sci. 1969, 31 (1), 116.
the dispersion forces between the phenolic π electrons and those of carbon basal planes.41 On the other hand, other experiments have shown that carbonylic groups enhance phenol adsorption because of a charge-transfer mechanism from the electron-rich carbonylic oxygen to the aromatic ring of the adsorbate.10,42 As expected, the donor acceptor mechanism works better for nitrophenols and chlorophenols because these molecules are better electron acceptors due to the electron-withdrawing effect of the nitro and chloro substituents. The role of surface carbonyl groups can be estimated from solids having the same pore size and pore distribution, same surface areas, and same chemical synthesis. This possibility is offered by the two derived carbons, that is, wCarbon and Carbon. They possess almost the same surface area, microporosity, mesoporosity, and total pore volumes, Table 2, while the IR spectra show the presence of carbonyl groups for wCarbon and not for Carbon. Comparing the 2,4,6-TCP uptake capacities of the two carbons from Figure 2, it is evident that wCarbon, which was treated with sulfuric acid, exhibits a higher uptake, 0.31 mg/m2, instead of 0.23 mg/m2 for Carbon without carbonyl groups on the surfaces. This result supports the donor-acceptor model which leads to increased phenol adsorption. 2. Gaseous Adsorption. The present materials were also examined for CH4, N2, and CO2 adsorption. Because of our gravimetric system pressure limitations (p < 2 MPa) the measurements were carried out at 253 K to be able to measure the full (i.e., up to p/p0 ≈ 1) CO2 adsorption isotherm (vapor pressure of CO2 at 253 K ) 1.95 MPa). It is expected that a similar picture will hold for roomtemperature adsorption but at much higher pressures (vapor pressure of CO2 at 298 K ) 6.44 MPa). The adsorption isotherms of these gases by Z-H+(uw)/C, uwCarbon, and Sug-H+/C are plotted in Figure 5. The CO2 adsorption isotherms from the three samples studied nicely depict their structural differences. SugH+/C reveals a totally microporous structure, which gives rise to a fully reversible Langmuir-like (type I according to IUPAC classification) adsorption isotherm. On the other hand, the adsorption isotherms of both Z-H+(uw)/C and uwCarbon reveal micro-, meso- and even macroporosities. Specifically, at low pressures there is a steep increase in the amount of the adsorbed gases reflecting surface coverage and micropore filling. As pressure increases the
Aqueous and Gaseous Adsorption
isotherms level off to a positive slope of almost a linear part indicating that multilayer formation takes place on the meso- and macropore pore walls as well as on the external surface area. As the bulk phase pressure approaches the vapor pressure an upward tail is observed, while upon desorption a clear type H3 hysteresis loop43 is revealed. Both features are fingerprints of sorptiondesorption in layered materials (e.g., clays), underlying the clay templating effect. Because the experimental temperature is far above the CH4 and N2 critical temperature and the pressure range is well beyond their critical pressures, the pertinent curves pertain practically to micropore filling and surface coverage phenomena. At low pressures (
