Langmuir 1996,11, 2151-2155
2151
Correlation between Surface Areas and Micropore Volumes of Activated Carbons Obtained from Physical Adsorption and Immersion Calorimetry M. T. Gonzalez, A. Sepulveda-Escribano, M. Molina-Sabio, and F. Rodriguez-Reinoso" Departamento de Quimica Inorganica, Universidad de Alicante, Apartado 99, E-03080, Alicante, Spain Received November 29, 1994. I n Final Form: March 7, 1995@ The heats of immersion of a series of activated carbons with increasing porosity into a number of liquids with different molecular sizes (benzene, 2,2-dimethylbutane and isooctane) have been measured at 303 K. The experimentalvalues have been used to calculatethe correspondingsurface areas (using a nonporous carbon (V3G) as reference) and micropore volumes. These parameters have been compared with those obtained from physical adsorption of Nz at 77 K and of the same hydrocarbons at 298 K. A good correlation has been found between the values obtained from both techniques provided no lack of accessibility of the liquid or vapor to the whole microporous structure of the carbon is present. 1. Introduction
Activated carbons are mainly used in adsorption processes, both in the gas and liquid phases, and their behavior as adsorbents strongly depends on their porous structure. The most extended technique for the characterization of the porous texture of activated carbons is the physical adsorption of gases and vapors. For microporous activated carbons, the adsorption process involves filling of the micropores before the surface of wider pores. So, the analysis of the adsorption data by means of the Dubinin theory and derived equations1 to obtain the micropore volume acquires a higher physical sense than the determination of surface areas using, for example, the BET methode2However,it is remarkable that the BET equation can mathematically fit the adsorption isotherms of type I, which are characteristic of microporous materials, providing values for the monolayer adsorption that never exceed the amount adsorbed at the plateau of the isotherm. Moreover, the BET surface area values, even being of doubtful physical sense in the case of superactivated carbon^,^ allow comparison of a number of carbons in a series as well as evaluation of the changes in the porous structure produced upon the activation process. The measurement of the heat evolved when a solid that is either degassified or partially covered by a n adsorbed film is brought into contact with a nonreacting liquid is the basis of the immersion calorimetry. This technique has been used for a long time for the characterization of surface properties of solids, microporous carbons among them, including surface areas, molecular sieve properties, and surface oxygen f u n c t i ~ n a l i t i e s .Recently, ~~~ Denoyel et al.4,6used immersion calorimetry to asses the surface areas and the micropore size distributions of two series of activated carbons. They obtained the specific enthalpy of immersion (per square meter of surface) for a carbon surface in different liquids by using a nonporous carbon
* Author to whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, May 15, 1995. (1) Dubinin, M. M. In Progress in Surface and Membrane Science Cadenhead, D. A., Ed.; Academic Press: New York, 1975; Vol. 9, pp 1-70. (2) Gregg, S. J.; Sing, K. S. W. In Adsorption Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982; pp 94-100. (3) Harrison, B. H.; Barton, S. S.;Dacey, J. R.; Sellors, J. R. J.Colloid Interface Sci. 1979,71, 367. (4)Denoyel, R.; Fernandez-Colinas, J.; Grillet, Y.; Rouquerol, J. Langmuir 1993,9, 515. (5) Stoeckli, H. F. Carbon 1990,28,1. @
black as a reference and assumed that the enthalpy of immersion is simply proportional to the surface area available to the immersion liquid, whatever the size and shape ofthe micropores. In this way, they obtained surface areas which correlated well with the values obtained from the application of the BET method to the nitrogen adsorption data, in spite of the fact that, as has been mentioned above, the concept of BET surface area is unrealistic when applied to microporous carbons. On the other hand, Stoeckli and Kraehenbuehl tried to relate the enthalpy of immersion and the parameters of the Dubinin's theory of micropore volume filling.7 They deduced that the enthalpy of immersion of a microporous solid into a liquid, the vapor of which is adsorbed according to the Dubinin-Radushkevich (DR) equation, is given by
In this equation, a and V,, are the thermal coefficient of expansion and the molar volume of the liquid, respectively, and -hi is the specific enthalpy of wetting (J m 3 of the external surface area. This equation shows that the enthalpy of immersion of a microporous material into a given liquid is a function of the micropore system, since Eo is related to an average pore size, and not only to the micropore volume, WO. This relation would permit the calculation ofthe external surface area, Se&,of a n activated carbon if the specific enthalpy of wetting (-hi) is known and Eo and WOare obtained from the physical adsorption of the corresponding vapor.8 However, for highly microporous carbons, the contribution of the microporosity to the total enthalpy of immersion is very large as compared with the contribution of the wetting of the external surface (-his,&),so the calculation of Sextfrom eq 1 may be affected by a n important error. The aim of this work was to evaluate the possibilities of immersion calorimetry as a tool to characterize microporous activated carbons, following the two approaches mentioned above to derive the parameters of the porous (6) Fernandez-Colinas, J.; Denoyel, R.; Grillet, Y.; Vandermeersch, J.; Reymonet, J. L.; Rouquerol, F.; Rouquerol, J. In Fundamentals of Adsorption; Mersman, A. B., Scholl, S. E., Eds.; Engineering Foundation: New York, 1989; pp 261-269. (7) Stoeckli, H. F.; Kraehenbuehl, F. Carbon 1981,19, 353. (8) Stoeckli, H. F.; Kraehenbuehl, F. Carbon 1984,22,297.
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2152 Langmuir, Vol. 11, No. 6, 1995
Gonzcilez et al.
texture of the solids (apparent surface area and micropore volume). The calculated values have been compared with those obtained from physical adsorption. 2. Experimental Section The series of activated carbons (series D) was prepared using olive stones which were carbonized in nitrogen at 1123 K for 2 h and later activated in carbon dioxide at 1098 K, with a flow rate of 100 cm3min-l, for different periods of time, as described in a previous paper;g the series covers the burn-off range from 8 to 70%. The experimental conditions used to determine the adsorption isotherms of nitrogen at 77 K and benzene, 2,2dimethylbutane, and isooctane at 298 K, together with the detailed analysis of the experimental data, have been reported in previous articles.1°-12 On the other hand, the nonmicroporous surface area of the activated carbons has been estimated by applying the a method to the benzene adsorption data, using a graphitized carbon black, V3G, as the reference material.13 The heats of immersion of the activated carbons and of the nonporous reference carbon into benzene, 2,2-dimethylbutane, and isooctane were determined a t 303 K with a conventional Tian-Calvet type differential microcalorimeter (Setaram, Model C80D). The samples (about 0.15-0.20 g of activated carbon and 0.4 g of V3G) were placed in a glass bulb with a brittle end and degassified at 523 K and Torr for 4 h, then the bulb was sealed and placed into the calorimeter cell containing 7 cm3 of the wetting liquid (Merck, analytical grade). Once the thermal equilibrium was achieved in the calorimeter block, the brittle end was broken and the liquid allowed to enter into the bulb and wet the sample, the heat flow evolution being monitored as a function of time. Thermal effects related with the breaking of the bulb and the evaporation ofthe liquid to fill the empty volume of the bulb with the vapor a t the corresponding vapor pressure were calibrated by using empty bulbs of different volumes.
3. Results and Discussion The series of activated carbons used in this study (series D) has been well characterized by physical adsorption of nitrogen a t 77 K, carbon dioxide and n-butane a t 273 K, and carbon dioxide, benzene, 2,2-dimethylbutane, and isooctane a t 298 K. These data have allowed a comprehensive description of the evolution of the porous texture of the activated carbons as a function of the activation degree, which has been reported in previous work^.^-'^ A brief summary ofthose findings, which is needed to develop the current work, is presented below. Activation of carbonized olive stones in carbon dioxide a t 1098 K produces, for low burn-off (e.g. carbon D-81, a microporous material with a narrow and homogeneous pore size distribution. As the activation proceeds, the creation of new micropores predominates up to 40-50% burn-off, whereas for carbons with an activation degree higher than 40-50% the predominant effect of gasification is the broadening of micropores. Figure 1 shows the evolution of the BET surface areas and the DR micropore volumes obtained from N2 adsorption at 77 K. The N2 DR plots for carbons with low burn-off show a long straight line with a slight upward deviation a t high relative pressures; however, as the burn-off increases, there are less experimental points defining the straight line and a t least two different values of WO(micropore volume) may be extrapolated.12 This is caused by the widening of the (9)Rodriguez-Reinoso,F.; Martin-Martinez, J. M.; Molina-Sabio, M.; Torregrosa, R.; Garrido-Segovia, J . J. Colloid Interface Sei. 1985,106, 315. (lo) Garrido, J.; Linares-Solano,A.; Martin-Martinez, J. M.; MolinaSabio, M.; Rodriguez-Reinoso, F.; Torregrosa, R. Langmuir 1987,3,76. (11)Garrido-Segovia, J.;Linares-Solano, A.; Martin-Martinez, J . M.; Molina-Sabio, M.; Rodriguez-Reinoso,F.;Torregrosa, R. J. Chem. SOC., Faraday Trans. 1 1987,83,1081. (12)Rodriguez-Reinoso, F.;Ganido, J.; Martin-Martinez, J . M.; Molina-Sabio, M.; Torregrosa, R. Carbon 1989,27,23. (13)Jaroniec, M.; Choma, J.; Rodriguez-Reinoso, F.;Martin-Martinez, J. M.; Molina-Sabio, M. J . Chem. SOC.,Faraday Trans. 1 1989,85,3125.
! 1,500
1,000
500
Burn-off (?h)
80 0
Figure 1. Evolution of microporevolumes and apparent surface areas, deduced from Nz adsorption, with burn-off (series D). Vo (cm3/g)
1
,
,
0.3
0.4
0.5
0
'0,
D-8
0.6
dlni" (nm)
Figure 2. Micropore size distributions deduced from physical adsorption of probe molecules (DRmethod).
micropore size distribution leading to different filling mechanisms in micropores of different sizes. The values of micropore volume which are plotted in Figure 1 for carbons D-52 and D-70 correspond to the extrapolation of adsorption data a t high relative pressures (second slope) and assigned to the whole microporosity filled by nitrogen.'O It can be seen in Figure 1 that the BET surface areas, as well as the micropore volumes of the less activated carbons, show a linear relationship with burnoff, whereas the micropore volumes of D-52 and D-70 increase more sharply with the activation degree. Consequently, when the main effect of the gasification process is the creation of microporosity, SBET and WOfollow the same tendency. However, when the micropores become wider, the increase of the micropore volume is more rapid than that of the surface area, as could be expected from geometrical considerations. The DR microporevolumes obtained from the adsorption of different gases and vapors are plotted in Figure 2 as a function of the molecular size of the adsorbate. The micropore volume of carbon D-8 is relatively low and decreases upon increasing the molecular size of the adsorbate, showing a clear molecular sieve effect for the largest molecules. The inaccessibility of 2,2-dimethylbutane and isooctane a s compared to smaller molecules like Nz or benzene and the long time required for the adsorption equilibrium to be reached confirm the narrow size of the micropores. As activation proceeds, there is a n increase in the micropore volume, basically due to the creation of new micropores but also, in some extent, due to the widening of the narrowest ones. There is still a molecular sieve effect for isooctane in carbon D-19, which disappears
Surface Areas and Micropore Volumes of Carbons
Langmuir, Vol. 11, No. 6, 1995 2153 Table 2. Apparent Surface Areas of the Samples (m2g-l)
-AHi (mJ/mZ)
BET
100 -
immersion calorimetry 2,2-DMB isooctane
sample
Nz (77 K)
benzene
D-8 D-19 D-34 D-52 D-70 V3G
647 797 989 1271 1426 62
754 917 1114 1402 1552 -
117 542 958 1192 1357 -
66 463 928 1243 1460 -
75 -
50 -
25
-
01 0
I
I
I
I
20
40
60
80
Burn-off (%)
Figure 3. Evolution of the areal enthalpies of immersion (mJ
m-2)into benzene, 2,2-&methylbutane,and isooctanewith burnoff (series D). Table 1. Experimental Enthalpies of Immersion of Activated Carbons (Series D) and V3G at 303 K into the Different Liquids (J g-l) sample D8 D19 D34 D52 D70 V3G
benzene 85.8 104.3 126.9 159.6 176.7 7.1
2,2-DMB 12.5 57.7 102.0 126.9 144.5 6.6
isooctane 7.2 50.7 101.6 136.1 160.0 6.8
for higher burn-offs. For carbons D-34 to D-70, all the adsortives give a similar value of micropore volume, this indicating that the micropores are, a t least, wider than the minimal dimension of isooctane (0.59 nm). 3.1. Surface Areas from Immersion Calorimetry. Table 1reports the enthalpies ofimmersion (-Mi(Jg-l)) of the activated carbons and the reference carbon black into the three liquids used: benzene, 2,2-dimethylbutane, and isooctane. The entalphies of immersion of the V3G reference into the three liquids are rather similar, about 7 J g-l; in the case of the activated carbons, the values obtained with benzene are systematically higher, even in the most activated samples, for which no lack of accessibility of the larger molecules to the whole microporosity can be expected, as indicated by data in Figure 2. On the other hand, the molecular sieve effect toward these molecules (2,2-dimethylbutane and isooctane) of carbons with low activation degree (D-8 and D-19) is clearly deduced from data in this table. The areal enthalpies of immersion (mJ m-2) into the three organic liquids are plotted versus the degree of activation (percent burn-off) in Figure 3. These values have been obtained by dividing the experimental enthalpies of immersion in Table 1between the N2 BET surface area of the samples. It is clear from this figure that the accessibility of 2,2-dimethylbutane and isooctane to the total porosity of samples D-8 and D-19 is very limited as compared with nitrogen, so the areal enthalpies of immersion are lower than those obtained with the reference material. However, the enthalpies of immersion per square meter are similar for the reference carbon and carbons with a higher degree of activation. By using a s a reference the values of areal enthalpies of immersion of the graphitized carbon black, the accessible surface areas for each liquid have been calculated, followingthe method
proposed by Denoyel et al.;4 they are presented in Table 2, together with the BET surface areas derived from nitrogen adsorption. As can be seen, except for carbons D-8 and D-19, for which the surface area accessible to 2,2-dimethylbutane and isooctane is much lower, very similar values are obtained with both techniques for activated carbons with larger burn-off not showing molecular sieve effect toward the larger molecules of the liquids. The minimal dimensions of nitrogen (0.36 nm) and benzene (0.37 nm) are rather similar and, consequently, they would have access to nearly the same range of porosity. Data in Figure 3 show that the areal enthalpies of immersion of the activated carbons into benzene are higher than that of the reference V3G carbon black, which is 114 m J m-2. This could be explained by either of the followingarguments (or both ofthem): (i)There is a higher carbon surface-benzene interaction in the activated carbons than in the reference. This approach has been rejected after using as a reference another nonporous standard, nongraphitized Vulcan 3,4and obtaining similar results. (ii)There is a n underestimation of the surface area of the activated carbons determined by nitrogen adsorption as compared with a n open surface like that of the carbon black. It can be seen in Figure 3 that this underestimation would decrease when the very narrow microporosity becomes broader, and the enthalpies of immersion vary from 133 m J m-2 for D-8 to 124 m J m-2 for D-70. The second approach has been recently proposed,4 and it is based on the fact that in very narrow micropores, of the width of one nitrogen molecule, the BET method takes into account only one of the walls of the micropore, thus underestimating the actual surface area, whereas the immersion calorimetry technique detects the interaction of the benzene molecule with both walls of the micropore. Thus, the narrower the microporosity of the carbon is, the higher the difference between the surface areas derived from nitrogen adsorption and from the enthalpies of immersion into benzene will be. This can be clearly seen in Table 2, where the surface areas obtained from both techniques are compared. Carbon D-8 has mainly micropores which are narrower than 0.56 nm, the minimal dimension of 2,2-dimethylbutane, and consequently, a high proportion of micropores are able to accomodate only one molecule of benzene or nitrogen. In this case, immersion calorimetry provides the surface area actually accessible to the adsorbates whereas the NOBET surface area is underestimated by the fundamentals of the method themselves. The surface areas derived from nitrogen adsorption and immersion into 2,2-dimethylbutane and isooctane are similar for carbons D-34, D-52, and D-70. This is due to the fact that the micropores in these carbons can accommodate a t least two molecules of nitrogen or a t least one molecule of the organic liquids. The presence of a residual narrow microporosity could be the origin of the higher surface area obtained with benzene. 3.3. Micropore Volumes from Immersion Calorimetry. It has been mentioned in the Introduction that,
Gonzalez et al.
2154 Langmuir,Vol. 11, No. 6, 1995
Table 4. Characteristic Energies (EO)and External Surface Areas (Sed)of the Activated Carbons, Obtained from Benzene Adsorption sample D-8 D-19 D-34 D-52 D-70
0;’ ,n
150 100-
50-A 0 0
. 50
100
A
2,2-DMB
0
Iro-octane
200
150
2,2-dimethylbutane,and isooctane.
Table 3. Affinity Coefficients (B), Thermal Expansion Molar Volumes (V,,,), and Specific Coefficients (a), Enthalpies of Wetting (-hi) of the Liquids Usedl6
p 1 1.12 1.71
isooctane
a
K-l) 1.24 1.44 1.24
23 27 33 53 83
2,2-dimethylbutane
benzene 250
Figure 4. Comparationbetween experimentaland theoretical enthalpies of immersion of the activated carbons into benzene,
liquid
Sext(m2 g-l)
27.4 26.3 25.1 23.0 21.7
Table 5. Micropore Volumes (cm3g-l) of the Activated Carbons Obtained from Physical Adsorption and Immersion Calorimetry
-AHi(exp) (J/g)
benzene 2,2-DMB
Eo (kJ mol-’)
V, (cm3mol-l)
-hi (J m-?
88.9 132.8 170.4
0.114 0.106 0.109
according to Stoeckli et a1.,5,7J4 the enthalpy of immersion of a microporous activated carbon into a liquid whose vapor is adsorbed followingthe DR equation is, once the enthalpy of wetting of the external surface (-h,Sext)has been substracted, directly proportional to EoWo, and it depends on the liquid used (eq 1).The validity of this relation for the series of activated carbons and the three liquids studied here is showed in Figure 4, where the theoretical enthalpies of immersion derived from eq 1, by using the parameters reported in Table 315and the Eo and W Ovalues obtained for each adsortive from the corresponding adsorption isotherm (DR method), are plotted versus the experimental enthalpies of immersion. The external surface areas, Sext,were obtained by the application of the a method to the benzene adsorption isotherm, with the nonporous V3G as reference,13and they are reported in Table 3. It can be seen in this figure that most of the points, whatever the liquid used, fit a straight line with a slope of unity. It follows from eq 1that the independent knowledge of Seaand -hi allows the possibility ofcalculating the product EoWo from the experimental enthalpy of immersion, but not each of these parameters separately. Consequently, to obtain the micropore volume from the data ofimmersion calorimetry in a given liquid, it would be necessary to make some assumptions about the likely value ofE0,16or to use the Eo value deduced from the physical adsorption of a reference adsorptive; however, it would be necessary to bear in mind that, in the case of bulky molecules, the effective Eo should be lower than for a small reference molecule, a s corresponds to the larger pores sizes seen by these molecules. In Table 5, the micropore volumes obtained with benzene, 2,2-dimethylbutane, and isooctane from physical adsorption (DR equation) and from immersion calorimetry are compared. These latter values have been calculated for the three liquids by using the and the external surface area characteristic energy (EO) (14) Bansal, D. C.; Donnet, J. B.; Stoeckli, H. F. In Active Carbon; Marcel Dekker: New York, 1988. (15)Reid, C. D.; Prausnitz, J. M.; Shenvood, T. K. In The Properties of Gases and Liquids; McGraw-Hill: New York, 1977. (16) Radeke, K.-H. Carbon 1984,22, 473.
isooctane
sample
ads.
imm
ads.
imm
ads.
imm
D-8 D-19 D-34 D-52 D-70
0.21 0.25 0.36 0.49 0.69
0.22 0.26 0.36 0.49 0.56
0.16 0.25 0.36 0.50 0.66
0.04 0.20 0.37 0.49 0.58
0.03 0.21 0.35 0.50 0.68
0.01 0.15 0.32 0.47 0.57
(Sext)deduced from benzene adsorption (Table 4) and the parameters characteristics of the different liquids reported in Table 3.15 In the case ofbenzene, the micropore volumes obtained from both techniques are coincident, except for carbon D-70, for which an important differenceis observed. Actually, the theoretical value of -Mi calculated for this carbon with eq 1 (214.6 J g-l) greatly exceeds the experimental one (176.7 J g-l). The three values for the micropore volume of D-70 obtained by immersion calorimetry are similar but lower than those obtained by physical adsorption. This can be explained by considering that this activated carbon possesses the widest micropore size distribution, with an important contribution of supermicropores. It is likely that this range of porosity contributes to the micropore volume deduced from the application of the DR equation to the adsorption isotherm in a more important way than to the total enthalpy of immersion, because it is filled by cooperative adsorption, only via adsorbate-adsorbate interactions.2 There are also important discrepancies in the micropore volumes calculated for D-8; whereas the values obtained with benzene are similar, much lower values are obtained from immersion calorimetry with 2,2-dimethylbutane and isooctane. This could be due to the choice of the Eo of benzene to calculate the micropore volumes with the other two molecules. Given the narrow microporsity of this sample (it is actually a molecular sieve for the two bulky molecules),the Eo corresponding to the microporosity seen by them will be much lower than that obtained with benzene, and this affects the calculated value of Wo, mainly when it is relatively small, as is the case for carbon D-8. For the other activated carbons in the series (D-19 still somewhat suffers the above mentioned influence of the choice of Eo), a very good correlation is obtained between the values calculated from both techniques. 4. Conclusions
The use of a nonporous reference allows the rapid determination of the surface area of a microporous activated carbon which is actually accessible to a given molecular probe by immersion calorimetry. The presence of narrow microporosity (able to accommodate only one nitrogen molecule) yields a higher value of surface area when measured by immersion into benzene than when nitrogen adsorption is used, and this is because the BET method applied to the Nz adsorption data does not take into account one of the walls of these narrow micropores, thus underestimating the actual surface area. On the
Langmuir, Vol. 11, No. 6, 1995 2155
Surface Areas and Micropore Volumes of Carbons other hand, micropore volumes obtained by the applicatiqn of the DR method to the adsorption isotherms ofbenzene, 2.2-dimethvlbutane. and isooctane well correlates with those obta&ed from immersion calorimetry following the equation proposed by Stoeckli et al.7 However, the determination of the micropore volumes by this method needs the previous knowledge of a series of parameters
(Eo, Sea)obtained from physical adsorption, so its practical application is limited.
Acknowledgment. Finantial support from DGICYT (Project No. PB9U0747) is gratefully acknowledged. LA940947C
