Langmuir 1994,lO, 2857-2859
2857
Entrance-Enriched Micropore Filling of n-Nonane Yohko Hanzawa, Takaomi Suzuki, and Katsumi Kaneko* Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263, Japan Received March 1, 1994. I n Final Form: June 6, 1994@ The interaction potential of an n-nonane molecule was calculated with the wall of slit-shaped graphitic micropores, with a width of 0.83 nm. The reason why n-nonane oRen blocks further adsorption near the entrance of micropores is discussed on the basis of the above calculation. Modification of the surfaces of activated carbon fibers (ACF) with titanium isopropoxide-triisostearate (TTS) enhanced markedly the low pressure adsorption of n-nonane at 303 K. This enhancement is associated with the quasi-equilibrium adsorption controlled by the micropore diffusion at the entrance of the micropore. A micropore filling mechanism is proposed in which the enrichment of n-nonane molecules near the hydrocarbon branches of TTS at the entrances of the Pores accelerates the micropore-diffusion facilitating the attainment of the adsorption equilibrium.
Introduction Micropore filling is the physical adsorption enhanced by overlapping of molecular potentials from opposite micropore walls; the strong admolecule-surface interaction gives rise to marked adsorption in the low pressure region.'g2 Activated carbons have a great amount of micropores, and they are effective for the adsorption of molecules in dilute vapor. However, the strong moleculesurface interaction often prevents completion of adsorption in whole of the micropores. That is, strongly adsorbed molecules near the entrance of micropores block further adsorption. In such a case, it is extremely difficult to attain the adsorption equilibrium, and then only a quasiequilibrium adsorption isotherm controlled by micropore diffusion can be ~ b s e r v e d .Consequently, ~ elimination of the diffusion limitation is necessary for improvement of adsorption characteristics of microporous solids even in the case of a strongly interacting vapor-solid surface system such as organic vapor and activated carbon. Activated carbon fibers (ACFs) have more uniform micropores than granulated activated carbons. The micrographitic structures and their adsorption properties have been widely i n ~ e s t i g a t e d .On ~ ~ the ~ other hand, n-nonane has been used for evaluation of smaller micropores with the aid of irreversible adsorption; a n n-nonane molecule interacts so strongly with the graphitic surface that n-nonane molecules near the entrance of micropores do not diffuse into the interior.' When the chemical environment of the external surface is changed to increase the affinity for the adsorptive molecules, the quasi-equilibrium adsorption limited by the micropore diffusion should be affected. In this paper, a marked enhancement of n-nonane adsorption in the low pressure region by surface modification is described. Experimental Section Cellulose-basedACF (KF-1500, Toyobo Co.) was used in this study. Dried ACF was immersed in a cyclohexane solution of
* Abstract published inAdvance ACSAbstracts, August 15,1994.
(1) Gregg, S. J.; Sing, K. S. W. In Adsorption, Surface Area and Porosity; Academic Press: London, 1982; Chapter 4. ( 2 )Everett, D. H.;Powl, J. C. J . Chem. Soc., Faraday Trans. 1,1976, 72,619. (3) Ruthven, D. M. In Principles of Adsorption and Adsorption Processes; Wiley-Interscience: New York, 1984; Chapter 5. (4) Kaneko, K.;Ishii, C.;Ruike, M.; Kuwabara, H. Carbon, 1992,30, 1057. (5) Wang, Z.-M.; Suzuki, T.; Uekawa, N.; Asakura, K.; Kaneko, K. J . Phys. Chem. 1992,96,10917 (6)Setoyama, N.; Ruike, M.; Kasu, T; Suzuki, T.; Kaneko, K. Langmuir lOOS,9,2619. (7) Gregg, S. J.;Tayyab, M. M. J . Chem. Soc., Faraday Trans. 1, 1978,74, 348.
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Figure 1. High-resolution a,plots of Nz adsorption isotherms: (0)ACF, ( 0 )ACF-0.74, (A)ACF-1.35.
titanium isopropoxide-triisostearate ('ITS), (CH3)2CHOTi[OC(O)(CH~)~~CH(CH~)ZI~* for 3 h at 333 K. TTS reacts with the surface hydroxyl groups of ACF.g The amount of TTS reacted with the ACF surface was spectrophotometrically determined using diantipyrylmethane after dissolution of the ashes of the burned sample in air. The 'ITS-modifiedACF is characterized by the coverage of the external surface with TTS. For example, ACF-0.74 denotes that the coverage is 0.74. In calculation the surface coverage,the molecular area ofTTS was presumed to be 3.2nm2from a geometricalestimation using van der Waals radii. The externalsurface area was attained from CQ plots (seebelow). The microporosity of l"S-modified ACFs was gravimetrically determined after pretreatment by Nz adsorption at 77 K with the use of computer-aidedequipment. The adsorption isotherm of n-nonane on the TTS-modifiedACF was determined at 303 K by use of a high-resolution volumetric equipment.1° The intermolecular potential @(z)of an n-nonane molecule in a slitshaped graphitepore was calculated as a function of the distance z between the molecule and the surface with the single-center approximation using the 10-4-3Steele's potential.ll
Results and Discussion Micropore Structural Change with TTS-Modification of Surface. The micropore structure was determined by the subtracting pore effect (SPE) method using the high-resolution Q plot,12 as shown in Figure 1. All high-resolution Q plots have the filling swing12 from (8) Hotta, Y.; Ozeki, S.;Suzuki, T.; Imai, J.; Kaneko, K.Langmuir 1991,7,2649. (9) Kaneko, K.;Ozeki, S.; Inouye, K. Colloid Polym. Sci. 1987,265, 1018.
(10) Sato, M.; Sukegawa, S.; Suzuki, T.; Hagiwara, S.; Kaneko, K. Chem. Phys. Lett. 1991,181,526. (11) Steele, W. A. Surf. Sci. 1973,36,317. (12) Kaneko, K.;Ishii, C. Colloids Surf. 1992,67, 203.
0 1994 American Chemical Society
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Letters modification is associated with enrichment of n-nonane molecules a t the entrance of the micropore due to favorable interaction of n-nonane with hydrocarbon chains of TTS. The amount ofthe n-nonane enrichment can be estimated from the adsorption isotherms shown in Figure 4. We will compare both isotherms of ACF and ACF-0.35. The equal amount of adsorption for both samples is obtained a t different relative pressures of 0.065 (for ACF) and 0.02 (for ACF-0.35), which are combined with by the broken lines. Although application of the relative pressure of 0.065 is necessary for the prescribed adsorption in the case of ACF, ACF-0.35 does not need such a high relative pressure. Application of PIP0 = 0.02 is sufficient for the
Langmuir, Vol. 10, No. 9, 1994 2859 adsorption by ACF-0.35. Thus the TTS modification increases the entrance concentration of n-nonane. The chemical entrance-modification of micropores can control even micropore filling of organic vapors. The detailed mechanism will be described in the future together with other data on organic vapors. "his entrancecontrolled micropore filling principle can be generally extended to other systems.
Acknowledgment. Financial support by the Science Research Grant from the Ministry ofEducation,Japanese Government, is greatly appreciated.
