Langmuir 1993,9, 3317-3319
n-Nonane Preadsorption as a Tool To Understand the Mechanism of COz Adsorption in Activated Carbons J. M. Martin-Martinez*,+and M. C. Mittelmeijer-Hazelegert Departamento de Qulmica Inorgbnica, Universidad de Alicante, Apartada 99, 03080 Alicante, Spain, and Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht, 166, 1018 WV Amsterdam, The Netherlands Received September 11, 1992. I n Final Form: July 9, 1993
Introduction Activated carbons are essentially microporous materi a l ~ the ~ - micropores ~ being classified in narrow-up to two adsorbate molecular size (a) dimension-and widebetween 2 and Ba-assuming two different mechanisms of adsorption depending on the relative sizes of adsorptive molecule and micropores, e.g., the micropore filling in narrow micropores, and the cooperative adsorption in wider micro pore^.^ The evaluation of total (narrow + wide) microporosity can be adequately assessed by applying the asmethod to the adsorption isotherms of N2/77 K, Ar/77 K, or n-C4Hlo/273 K.k7 However, the evaluation of the narrow and wide micropore volumes independently is not a simple task, mainly in carbons with a wide micropore size distribution. Usually,narrow microporosity is evaluated by n-nonane preadsorpti~na~ or from the Dubinin-Ftadushkevich equation applied to the adsorption isotherms of C02 at 273298 K.'O Admitting that C02 is mainly retained in pores smaller than 2a at these temperatures, its adsorption might be produced by either a micropore filling and/or surface coveragemechanism.'O For some authors,lJ2C02 adsorbed at around room temperature (273 or 298 K) only could be as a monolayer covering the micropore walls (because the critical temperature of C02 is 303 K, the adsorption potential in the micropores might not be strong enough to give condensation at the adsorption temperature), whereas for others,l3J4 C02 adsorbed in fine micropores is in a liquid or even closer to a solid phase. It is well-known8~9that n-nonane is retained by different adsorption potentials depending on the micropore sizes: the narrower the microporosity,the higher the temperature needed to remove n-nonane from micropores. Therefore,
* Author to whom correspondence should be addressed.
t Universidad
de Alicante. University of Amsterdam. (1)Bansal,D.C.; Donnet, J. B.; Stoeckli,H. F. Actiue Carbon; Marcel
Dekker: New York, 1988. (2)Smisek, M.; Cerny, S. Actiue Carbon;Elsevier: Amsterdam, 1970. (3)Rodrlguez-Reinoso,F.;Martfn-Martfnez, J. M.; Molina-Sabio,M.; PBrez-Lledb. I.: Prado-Burrmete,C. Carbon 1985,23,19. (4)Carrott, P. J. M.; Shg, K. S. W. Characterization of Porous Solids-COPS I; Unger, K. K., Rouquerol, J., Sing, K. S. W., Kral, H., Us.; Elsevier: Amsterdam, 1988;pp 77-87. (5) SellBs-PBrez,M. J.; Martfn-Martfnez,J. M. J. Chem. SOC.,Faraday Trans. 1 1991,87,1237. (6) Sell&-Pbrez, M. J.; Martfn-Martinez, J. M. Fuel 1991,70,877. (7)Sellbs-PBrez, M. J.; Martin-Martinez, J. M. Carbon 1992,30,41. (8)Linares-Solano,A,; L6pez-GonzBlez,J. D.; Martfn-Martinez,J. M.; Rodrlguez-Reinoso, F. Adsorpt. Sci. Technol. 1984,1 , 123. (9)Rodriguez-Reinoso,F.;Martfn-Martfnez, J. M.; Molina-Sabio,M.; Torregrosa, R.; Garrido, J. J. Colloid Interface Sci. 1985,106, 315. (10)Garrido, J.; Linares-Solano,A.; Martfn-Martfnez,J. M.; MolinaSabio, M.; Rodrfguez-Reinoso,F.; Torregrosa, R. Langmuir 1987,3,76. (11)Lamond, T. G.; Marsh, H. Carbon 1964,1 , 281. (12)Marah, H.;Siemieniewska,T. Fuel 1965,44,355. (13)Walker, P. L., Jr.; Patel, R. L. Fuel 1970,49,91. (14)Mahajan, 0.P. Coal Structure; Academic Press: London, 1982; p 51.
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stepwise removing n-nonane with increasing temperature will selectively leave unfilled micropore ranges of progressively smaller size. In this way, CO2 adsorption on carbons containing adsorbed n-nonane can be used to follow the changes (if any) in its mechanism of adsorption, allowing a relationship between micropore size and the adsorption mechanism of C02 to be established. The original contribution of this paper is the combined use of the n-nonane preadsorption technique and COz/ 273 K adsorption to differentiate the type of mechanism by which the adsorption of CO2 in micropores of different molecular sizes takes place. Experimental Section Because of the excellent properties and our wide knowledge of the porous texture of activated carbons prepared from lignocellulosic materials,'6J6 plum stone was the raw material selected to prepare the carbons by using COz activation16in order to produce a slow and controlled development of microporosity. Plum stones were conveniently crushed and sieved in order to obtain 0.5-0.8 mm particle size in the activated carbons and treated with 1 N HzSOl for 6 h; the raw material was then extensively washed with tap water until neutral pH and finally washed twice with bidistilled water, filtreated, and allowed to dry overnight at room conditions. The dried material was carbonized in nitrogen (100 cm*min-1) at 1173K for 2 h; heating rate was 5 deg min-I. The carbonization process was maintained at 673 K for 15min to prevent the formation of tars, which block the entrances of narrow pores in the carbon. Carbonization yield was 23 w t %, as in most of lignocellulosic materials.15J6 The carbonized material was subsequently activated in COZ (100 cm* min-l) at 1133 K between 24 and 75 h to obtain three activated carbons with 23 (carbon X),52 (carbon Y),and 71% burn-off (carbon Z). The activated carbons were characterized by adsorption of Nd77 K, Cod273 K, and n-nonane preadsorption using a fully automated Sorptomatic 1800 (Carlo Erba) to determine the adsorption-desorption isotherms. Samples of 0.2-4.4 g of carbon were placed into the adsorption cell and outgassed at 573 K for 3 h under vacuum (10-9 Torr; 1Torr = 133.33 kPa) prior to the adsorption measurements. The n-nonane preadsorption was carried out as follows. Once the Nd77 K or Cod273 K adsorption isotherm of the activated carbon was obtained, carbon was outgassed again at 573 K for 3 h under vacuum (10-9 Torr) and h r w a r d the adsorption cell was cooled down to 273 K. A portion of 3 mL of liquid water-free n-nonane (98% minimum purity, Merck AR) was placed on the upper part of the adsorption cell in contact with the stopcock of the cell; afterward, the stopcock was carefully opened to allow around 2 mL of n-nonane into the cell, and then the stopcock was closed; a noticeable evolution of heat was observed when the n-nonane was put in contact with the outgassed activated carbon. Finally, the adsorption cell was placed in the outgassing unit of the adsorption apparatus and evacuated at room temperature until constant weight, the outgassing time of n-nonane being a function of the carbon. This sequence was repeated upon increasing the outgassing temperature of n-nonane up to 398, 473, 573, and 723 K after evacuating n-nonane at every temperature, the Nd77 K or Cod273 K adsorption isotherms were obtained. Nd77 K or Cod273 K adsorption isotherms, obtained after completelyremovingthe n-nonane at 723 K, agreed well with those of the original carbons, indicating that the adsorption and desorption of n-nonane do not modify the pristine porous texture.
Results and Discussion There is a development of the microporosity (both in volume and size distribution) with increasingburn-off from carbon X to Z. The amlication of the DR eauation to of
(15)Rodriguez-Reiioso,F.; Linarea-Solano,A. Chemistry andPhysics Carbon; Thrower, P., Ed.; Marcel Dekker: New York, 1989;Vol. 21,
P 1. (16)Torregrosa, R.PhD Dissertation, University of Alicante. Spain, 1984.
0 1993 American Chemical Society
3318 Langmuir, Vol. 9, No. 11,1993
Notes
Table I. Micropore Volumes and Pore Volume Accessible to Nz ( c d gl)of the Activated Carbons pore volume accessible to Na after evacuation Vo-DR" of n-nonaneatc carbon Na/77K COd273K V298K 398K 473K 573K X
Y
z
0.30 0.62 0.88
0.34 0.63 0.70
0.26 0.57 0.74
0.32 0.65 0.76
0.29 0.34 0.28
0.22 0.14 0.14
0.04 0.03 0.07
a Micropore volume calculated by
applying the Dubinin-Raduehkevich equation. bAmount of n-nonane (cm9 g-') retained in the carbon. c Calculated by subtracting the amount adsorbed at p / p o= 0.7 in the N d 7 7 K adsorption isotherms.
&/77 K and Cod273 K adsorption isotherms gives the micropore volumes (VO)included in Table I. Carbon X exhibits a narrow microporosity of 0.3-0.4 nm in size, VO(N2) < V0(CO2).l7 Carbon Y has micropores smaller than 1 nm in size, Vo(N2) = V0(CO)2.l7 Carbon Z shows a welldeveloped microporous texture with micropore sizes between 1and 2 nm, Vo(N2) >> Vo(C0)z. Therefore, these three carbons cover a wide range of micropore size distributions, and present the same features as most of the CO2-activated carbon series prepared from lignocellulosic p r e c u r ~ o r s . ~ J ~Considering J~J~ that the minimum dimension of n-nonane (around 0.43 nm) is greater than that of N2 or C02 (0.30.34nm), in carbona with micropore size smaller than 0.43 nm n-nonane will not fill them but block the entrances of the slit-shaped micropores.lg Thus, the volume of liquid n-nonane retained in the micropores of carbon X (0.26 cm3 g-9 is smaller than Vo(N2) or VO(C02)(Table I); however,the micropore volumescalculated as the difference between the amount of N2 adsorbed in the isotherms (taken at a relative pressure of 0.7) before and after n-nonane preadsorption at 298 K, agreed well with Vo(N2) (Table I). Furthermore, the narrow microporosity in carbon X is confirmed by the fact that n-nonane is suddenly removed from the micropores at temperatures greater than 473 K. The widening of the microporosity facilitates the filling of the micropores (instead of its blockage) with n-nonane, so in carbony there is an agreement between the micropore volumes calculated from (i) the amount of n-nonane adsorbed in micropores, taken as the difference between the amount of N2 adsorbed before and after n-nonane preadsorption at 298 K at a relative pressure of 0.7, and (ii) the DR equation applied to the N2 isotherm of the originalcarbon (Table I). Important amounts of n-nonane are removed at 398 K, the stepwise increase of temperature producing a gradual removal of residual n-nonane from the micropores because the relatively wide micropore size distribution in carbon Y. Due to the well-developed microporosity of carbon Z, there is no agreement between the micropore volume VO(N2) and Vo(CO2), from the DR equation, whereas the microporevolume (i) Vo(CO2),(ii)the volume of n-nonane retained in the micropores,and (iii) the difference between the volume of N2 adsorbed in the isotherms obtained before and after n-nonane preadsorption at 298 K agree quite ~ell.&~J~ It has been previously establishedlOthat C02 adsorption at 273 K provides the volume of the narrower microporosity of activated carbons. It has also been shown16 that the (17) Mittelmeijer-Hazeleger,M. C.; Martfn-Martfnez, J. M. Carbon 1992, 30, 696. (18)Siemieniewska,T.;Tomkow, K.; Kaczmarczyk, J.; Albininak, A.; Grillet, Y.; Franpis, M. Characterization of Porous Solids-COPS II. Alicante, May 1990; pp 410-412. (19) Martfn-Martfnez, J. M.; McEnaney, B.; Molina-Sabio,M.; Rodrlguez-Reinoso,F. Carbon 1986,24, 255.
COJ273 K adsorption. Carbon X. n-nonane preadsorption 5 F
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PIP' Figure 1. Cod273 K adsorption isotherms of activated carbons with n-nonaneremovedat differenttemperatures: (a,top) carbon XI(b, middle) carbon Y, (c, bottom) carbon Z; ( 0 )original, (+) 298 K, ( 0 ) 398 K,(A)473 K,( 0 )573 K,(v)723 K.
enlargement of the narrow microporosity produces a change in the shape of the COZ adsorption isotherms, carbons with anarrow microporosityshowingquite curved adsorption isotherms, whereas wide microporosity produces more linear COZadsorption isotherms. According to Figure 1,the C02 adsorption isotherm for carbon X is completely curved, whereas for carbon Z becomes almost linear at relative pressures greater than 0.015. In our
Langmuir, Vol. 9, No. 11, 1993 3319
Notes
opinion, this change of shape in the CO2 isotherms is reflecting a change in the mechanism of adsorption of C02 in the micropores probably associated to a decrease in the adsorption potential. In this sense, Ismail and Mahowaldm have found linear C02 adsorption isotherms for nonporous graphitized carbons and a curvature at low relative pressures for ungraphitized carbons; this curvature was ascribed by the authors to the presence of microporosity. Furthermore, by extrapolating the conclusionsof Ismail and Mahowald20to our ungraphitized activated carbons, the widening of the microporosity, which puts farther apart the micropore walls with the consequent decrease of the adsorption potential, will gradually allow the C02 to be adsorbed by a surfacecoverage, instead of micropore filling, mechanism. Following this argument,in activated carbons with a narrow microporosity the adsorption potential will be very high and the micropore filling process with COZ molecules will be produced. This hypothesis can be confirmed by following the changes in the shape of C02 adsorption isotherms in the carbonswhose microporosity is partially accessible to CO2. In this paper the progressive accessibility of C02 to the narrower microporosity of the carbons is produced by stepwise removing n-nonane at different temperatures. At this point, it is important to state that C02 does not diffuse or alter adsorbed n-nonane. In a recent paper Stoeckli et al.21showed by immersion calorimetry that a strong interaction between n-nonane and the micropore walls is established. Thus, a displacement of n-nonane from micropores by C02 at 273 K is not expected. Figure 1shows the Cod273 K adsorption isotherms of activated carbons X, Y,and Z obtained by removing n-nonane at different temperatures. For carbon X (Figure la) the increase of temperature to remove n-nonane (narrower microporosity becomes progressively accessible to C02) producesa change in shape, from linear to curved, in the CO2 isotherms. Because at temperatures below 473 K all the microporosity is blocked by n-nonane (Table I), at these temperatures C02 adsorption only could take place in the external surface of the carbons and therefore linearshaped isotherms are found. When narrow microporosity is accessible to CO2, curved-shaped isothermsare obtained, the narrower the accessiblemicroporosity the more curved the isotherms. Furthermore, because the micropore size (20) Ismail, I. M.K.; Mahowald, M.A. X X Biennial Conference on Carbon; Americal Carbon Society University Park, PA, 1991; p 70. (21) Stoeckli,F.; Huguenin, D.; Rebatein, P. J. Chem. SOC.,Faraday Trans. 1 1991,87,1233.
of carbon X is 0.3+4nm, the curved-shaped C02 isotherm can be associated to micropores of one adsorbate molecular dimension size. Parts b and c of Figure 1 show linear-shaped C02 adsorption isotherms for carbons Y and Z, respectively, having these carbons an important fraction of micropores accessible to C02 at temperatures below 473 K. Carbon Y has micropores of 1nm in size which can allow two to three C02 molecules to be adsorbed on it, so far carbon heated below 473 K, a surface coverage of C02 will be produced. Only by removing n-nonane from micropores of one CO2 molecular size (suddenly produced at 573 K), the adsorption on such micropores will contribute to the curvature of the isotherm by a micropore-filling mechanism. For carbon Z (Figure IC)the situation is even more clear. Therefore, the curvature in the Cod273 K adsorption isotherms is mainly produced by adsorption in micropores of molecular size dimensions. C02 in micropores of 2-3 molecular diameter sizes prefers to be adsorbed by surface coverage. The assessmentof micropore size and NdT7 K isotherm in shape was also recently studied by Kakei et activated carbon fibers and molecular sieving carbons. In micropores having three or more molecular size dimensions, there is a surface coverage mechanism. With an increase of the relative pressure, a filling of such micropores is produced and, therefore, there is a change in the slope of the N2 isotherms. These fidings are useful to support the results given here.
Conclusions The adsorption of Cod273 K in micropores of one molecular dimension is produced by a micropore-filling mechanism, which is associated to curved-shaped isotherms. The widening of the micropores (by increasing the percentage in burn-off of the activated carbon) gradually favored the surface coverage by C02 in micropores with 2-3 molecular size dimensions. The combined use of C02 adsorption and n-nonane preadsorption allows one to follow the changes in the mechanism of adsorption in narrow micropores of activated carbonswith increasing bum-off.
Acknowledgment. The authors wish to thank Richard Brouwer for his assistance in the experiments. (22) Kakei,K.;Ozeki, S.;Suzuki,T.;Kaneko,K. J. Chem.Soc.,Faraday
Trans. 1 1990,86,371.
