Langmuir 2001, 17, 4967-4972
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Study of Diethyl Ether Adsorption on Activated Carbons Using IGC at Finite Concentration Issa I. Salame‡ and Teresa J. Bandosz*,† Department of Chemistry, The City College of New York, and The Graduate School of The City University of New York, New York, New York 10031 Received February 5, 2001. In Final Form: May 23, 2001 Two samples of activated carbon of wood origin were oxidized using ammonium persulfate. The samples were characterized using sorption of nitrogen and Boehm titration. Then, the adsorption of diethyl ether was studied on the initial samples and their oxidized counterparts by means of inverse gas chromatography at finite concentration. Adsorption isotherms were obtained from the chromatographic peaks using a characteristic-peak elution method. Then, the isotherms at temperatures between 393 and 433 K were used to calculate the isosteric heats of adsorption. The results showed a difference in the uptake of diethyl ether depending on the porosity of the sample and its surface chemistry. Analysis of the heats of adsorption revealed that the diethyl ether molecules are adsorbed on the carbon surface via two different adsorption mechanisms. First, hydrogen bonding to functional groups in narrow pores significantly contributes to the adsorption. Second, pore sizes govern the adsorption uptake as a result of interactions of the hydrocarbon moiety with the pore walls.
Introduction Activated carbons are popular adsorbents. They are used to remove and separate various molecules from gaeous, aqueous, and nonaqueous media. Their most common applications are in gas purification, solvent recovery, catalysis, and fuel cells.1,2 It is well-known that activated carbons have heterogeneous surfaces from the point of view of both surface chemistry and porous structure. This structural heterogeneity is the result of the presence of pores ranging from a few to a few hundred angstroms. It is believed that they are slit-shaped.3 The surface chemistry of the carbon is the result of the presence of heteroatoms such as oxygen, nitrogen, hydrogen, and phosphorus.4,5 The contents of these elements and the amounts and types of groups formed depend on the origin of the carbon and the method of activation.6 The presence of heteroatoms such as oxygen, which can form ketones, carboxyls, phenols, ethers, and lactones; nitrogen in the form of amines; and phosphorus as phosphates determine the acidity or basicity of the activated carbon.4,5 The presence of heteroatoms also has a significant effect on the adsorption of polar species.7 Moreover, heteroatoms can catalytically affect the conversion of adsorbed species as in the case of hydrogen sulfide adsorption,8,9 or they * Author to whom correspondence should be addressed. Tel.: (212) 650-6017. Fax: (212) 650-6107. E-mail: tbandosz@ scisun.sci.ccny.cuny.edu. † Department of Chemistry, The City College of New York. ‡ The Graduate School of The City University of New York. (1) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988. (2) Smisek, M.; Cerny, S. Active Carbon; Elsevier: Amsterdam, 1970. (3) Donnet, J. B.; Papirer, E.; Wang, W.; Stoeckli, H. F. Carbon 1994, 32, 183. (4) Boehm, H. P. In Advances in Catalysis; Academic Press: New York, 1966; Vol. 16. (5) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6. (6) Marsh, H., Heintz, E. A., Rodriquez-Reinoso, Eds. Introduction to Carbon Technologies; University of Alicante: Alicante, Spain, 1997. (7) Avgul, N. N.; Kiselev, A. V. In Chemistry and Physics of Carbon; Walker, P. L. J., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6. (8) Adib, F.; Bagreev, A.; Bandosz, T. J. J. Colloid Interface Sci. 1999, 216, 360. (9) Adib, F.; Bagreev, A.; Bandosz, T. J. Langmuir 2000, 16, 1980.
can create obstacles to the physical adsorption of nonpolar molecules.10 Diethyl ether, with the chemical formula (C2H5)2O, is a polar molecule that can interact with the carbon surface via dispersive interactions of its hydrocarbon moiety, two ethyl groups. It can also be hydrogen bonded to oxygencontaining surface functional groups such as carboxylic and phenolic compounds. In addition, ether is capable of donating a pair of electrons from the oxygen lone pairs and, thus, interacting with the electron-pair receptors on the surface as a Lewis base.11 The heat of diethyl ether adsorption on carbon black was reported to be about 36 kJ/mol.7 When the adsorption of various ethers on amorphous nitrogenated carbons was studied, the desorption energy of diethyl ether was found to be between 50 and 60 kJ/mol.11 On the other hand, Guiochon et al. reported that the energy of adsorption of ether on silica has two specific regions, one at 46 kJ/mol and the other at 57 kJ/mol, that can be related to interactions with different surface sites.12 The objective of this paper is to study the adsorption of diethyl ether on two activated carbons of wood origin. The samples differ significantly in porosity and surface chemistry as a result of their preparation by different activation methods.13 These differences allow us to study the effects of oxygen-containing surface groups and pore sizes on the adsorption process. The mechanism of adsorption is analyzed on the basis of adsorption uptakes and the energetics of adsorption expressed as isosteric heats of adsorption. Experimental Section Materials. Two activated carbons of wood origin supplied by Westvaco were used for this study. The first carbon, WVA 1100, is referred to as WVA. It is obtained using phosphoric acid activation at 900 K.14 The second carbon, designated as UMC, (10) Bandosz, T. J.; Jagiello, J.; Schwarz, J. Langmuir 1993, 9, 2518. (11) Paserba, K.; Shukla, N.; Gellman, A. J. Langmuir 1999, 15, 1709. (12) Guiochon, G.; Stanley, B. J. Langmuir 1995, 11, 1735. (13) Salame, I. I.; Bandosz, T. J. Ind. Eng. Chem. Res. 2000, 39, 301. (14) Jagtoyen, M.; Derbyshire, F. Carbon 1993, 31, 1185.
10.1021/la010188f CCC: $20.00 © 2001 American Chemical Society Published on Web 07/06/2001
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Langmuir, Vol. 17, No. 16, 2001
Salame and Bandosz In the characteristic-point elution method,21-27 the total amount of solute adsorbed on the solid support is calculated from the formula
V)
FC k m
∫ (t - t ) dh h
0
0
(1)
where V is the amount adsorbed; FC is the corrected flow of helium gas through the column;28 m is the weight of the adsorbent; t and t0 are the retention times of the adsorbent and the nonadsorbed species (methane), respectively; and k is a proportionality constant between the height of the peak, h, and the corresponding concentration at that particular height, Ci. Using this equation, one can divide the chromatographic peak into i slices that corresponds to i pressures and injected amounts,13,28,29 and thus
Figure 1. Principles of the characteristic-point elution method used to calculate adsorption isotherms. is a developmental adsorbent manufactured by KOH activation of WVA 1100 at 1300 K.15 Both carbons were oxidized with ammonium persulfate.16,17 Prior to any analysis, the samples were washed in a Soxhlet apparatus to remove both water-soluble species and excess oxidizing agent (when oxidation was applied). Samples obtained after oxidation are referred to as WVA-O and UMC-O, respectively. Methods. Boehm Titration. The Boehm titration method was used to assess oxygen-containing surface groups.4,18 In this method, 1 g of carbon is placed in 50.0 mL of the following 0.05 N solutions: sodium hydroxide, sodium carbonate, sodium bicarbonate, and hydrochloric acid. The vials are then sealed and shaken for 24 h, and the carbon suspension is filtered. Five milliliters of each filtrate is titrated with acid or base (HCl or NaOH). The number of acidic sites is calculated using the assumptions that NaOH neutralizes carboxylic, phenolic, and lactonic groups; Na2CO3 neutralizes carboxylic and lactonic groups; and NaHCO3 neutralizes only carboxylic groups. The number of basic sites is calculated from the amount of HCl that reacted with the carbon. Sorption of Nitrogen. Nitrogen isotherms were measured using an ASAP 2010 instrument (Micromeritics) at 77 K. Before each experiment, the samples were heated at 393 K and outgassed at this temperature under a vacuum of 10-5 Torr. The isotherms were then used to calculate specific surface area; micropore volume, Vmic; volume in pores smaller than 10 Å, V
