Langmuir 1987, 3, 331-335 sorption theories and the applicability of continuing to extend such approaches to other microporous solids that are less well characterized with respect to structure. Acknowledgment. Dr. Jorge Soto kindly provided Henry’s law constant values that he had compiled from original sources. Dr. Robert Pierotti provided useful in-
331
formation and engaged in helpful discussions of this work. The University of Tennessee a t Chattanooga Center of Excellence for Computer Applications and the University of Chattanoga Foundation both provided funding for this project. Registry No. Ar, 7440-37-1; Kr, 7439-90-9; Xe, 7440-63-3.
Adsorption of Nitrogen and Neopentane Vapor by Microporous Carbons? R. Adrian Roberts, Kenneth S. W. Sing,* and Vijai Tripathit Department of Chemistry, Brunel University, Uxbridge, Middlesex, UB8 3PH England Received September 15, 1986. In Final Form: January 21, 1987 Adsorption isotherms of nitrogen (at 77 K) and neopentane (at 273 K) have been determined on five microporous carbons. Application of the a,-method of isotherm analysis has provided supporting evidence for the occurrence of two stages of micropore filling: (a) a primary process involving enhanced adsorbent-adsorbate interactions in pores of molecular dimensions and (b) a cooperative process taking place in wider micorpores. On this basis it has been possible to make a tentative assessment of the range of effective pore size in each carbon. Introduction Although a number of attempts have been made in recent years to assess the micropore size distribution from physisorption data, the mechanisms involved in the filling of micropores are still far from c1ear.l According to the IUPAC classification of pore size: micropores are defined as pores with widths not exceeding -2 nm. Theoretical calculations3 and recent experimental measurements4 of adsorption energies have indicated, however, that enhancement of the gas-solid interaction energy becomes quite small as the pore width is increased to more than a few molecular diameters. To account for this apparent discrepancy in the upper limit of micropore size, it has been ~uggestedl7~~~ that the wider micropores are filled by a different mechanism, which appears to be a cooperative process. It is consistent with this explanation that the differential enthalpies and entropies of adsorption for nitrogen and argon have been found4to exhibit different features corresponding to the two stages of pore filling. Nitrogen is still and most popular adsorptive for the investigation of micropore filling’ and, apart from molecular probe measurements: few attempts have been made to utilize more bulky molecules such as neopentane (2,2dimethylpropane). In principle there are a number of advantages to be gained by using neopentane (e.g., its nonpolar character and globular shape) and there would appear to be considerable scope for its applicationparticularly with the aid of the a,-method of isotherm analysi~.~ The work reported here was carried out as part of a long-term investigation of a number of well-characterized microporous carbons. The main objective was to make a comparative study of neopentane and nitrogen adsorption t Presented at the “Kiselev Memorial Symposium”,60th Colloid and Surface Science Symposium, Atlanta, GA, June 15-18,1986; K. S. W.Sing and R. A. Pierotti, Chairmen. Permanent address: Defence Research & Development Establishment, Gwalior-2, India.
*
0743-7463/87/2403-0331$01.50/0
and extend the application of the a,-method for the assessment of microporosity. Experimental Section Five microporous carbons were selected as representative adsorbents having different pore structures. Carbosieve (S) is a polymer-basedmolecular sieve carbon manufactured by Supelco, Bellefonte, PA. AX21 is a petroleum pitch-based active carbon manufactured and supplied by the Anderson Development Co, MI. Three samples of charcoal cloth were specially prepared from viscose rayon cloth by carbonization in N2 at 850 “C followed by activation in COz at the same temperature.s JF005 is a lowburn-off cloth of relatively low BET area and pore volume, while JF143 and JF517 were prepared at higher burn-off and under conditions designed to give higher areas and pore volumes. The nitrogen isotherms were determined at 77 K by means of a Carlo-Erba Series 1800 Sorptomatic,which had been calibrated with a manual volumetric apparatus. A Datametrics Barocel transducer gauge was employed for the measurement of low pressures. The neopentane isotherms were determined at 273 K with the aid of a CI Electronics microbalance and a Texas
(1)Gregg, S.J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic: New York, 1982; Chapter 4. (2) Sing, K. S.W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985,57 (4), 603. (3) Everett, D.H.;Powl, J. C. J. Chem. SOC.,Faraday Trans. 1 1976, 72, 619.
(4) Atkinson, D.; Carrott, P. J. M.; Grillet, Y.; Rouquerol, J.; Sing, K. S . W. “Fundamentals of Adsorption”, Proc. Eng. Foundation Conf., 1986, in press. (5) Sing, K. S.W. In Principles and Applications of Pore Structural Characterisation; Haynes, J. M., Rossi-Doria, P., Eds.; J. W. Arrowsmith Ltd.: Bristol, 1985; p 1.
(6) Breck, D.W. Zeolite Molecular Sieues Structure, Chemistry, and Use; Wiley: New York, 1974; p 637. (7) Sing, K. S. W. In Surface Area Determination; Everett, D. H., Ottewill, R. H., Eds.; Butterworths: London, 1970; p 25. (8)Capon, A.; Freeman, J. J.; McLeod, A. I.; Sing, K. S. W. Proc. London Int. Carbon Graphite Conf., 6th 1982,154.
0 1987 American Chemical Society
332 Langmuir, Vol. 3, No. 3, 1987
Roberts et al.
NEOPENTANE
..k-----"i NITROGEN
1.4
- -
1.2
v
-
NEOPENTANE c "
1
"
'm
.
0
>
9 1.0
>
:I
0.8
0.2
-
1
I
1
, 0.2
0.4
06
0.8
P/ Po
0.2
[ , , , , , , , , , I 0.2
04
PIP"
08
Figure 3. Adsorption isotherms of neopentane and nitrogen on charcoal cloth JF143 open symbols,adsorption; closed symbols,
desorption.
Figure 1. Adsorption isotherms of neopentane and nitrogen on AX21: open symbols, adsorption; closed symbols, desorption. I
harcoal Cloth JF005. NITROGEN
I
t I
d
I 0.2
0.4
06
0.8
PIP"
Figure 4. Adsorption isotherms of neopentane and nitrogen on charcoal cloth JFOO5 open symbols, adsorption; closed symbols,
desorption.
0-2
0.4
PIP" o-6
0.8
Figure 2. Adsorption isotherms of neopentane and nitrogen on charcoal cloth JF517: open symbols,adsorption; closed symbols, desorption.
Instrument precision pressure gauge. Adsorbent samples were outgassed at 250 O C (523 K)for 16 h to a residual pressure of -1Od torr.
Results and Discussion The adsorption isotherms of neopentane and nitrogen are displayed in Figures 1-5. To facilitate comparison between the corresponding isotherms, the amout of vapor adsorbed is expressed as the equivalent volume of liquid adsorptive-but of course it cannot be assumed that the adsorbate actually had liquidlike properties. The type I character of the isotherms is indicative of the microporous
nature of all the carbons, although it is evident that there were significant differences in their pore-size distributions. Thus, the saturation uptakes approximately conform to the Gurvitsch rule' in only two cases (samples AX21 and JF517). Some molecule sieving has been exhibited by the other three carbons, resulting in a lower adsorption capacity for the more bulky molecules of neopentane than for nitrogen. Low-pressure hysteresis is a distinctive feature of the neopentane isotherms on JF005 and Carbosieve (Figures 4 and 5). This behavior has been shown to be reproducible, and similar resulta have been obtained recently with certain other microporous adsorbents. Low-pressure hysteresis may be due to either the swelling of the adsorbent and irreversible intrusion of adsorbate molecules into slit-shaped channelsQor the activated entry of the neo(9) Bailey, A.; Cadenhead, D. A.; Everett, D. H.; Miles, A. J. Tram. Faraday SOC.1971,67, 231.
Langmuir, Vol. 3, No. 3, 1987 333
Adsorption on Microporous Carbons
om
CARBOSIEVE.
p4
0.1
PIP0 I
I
0.8
1
0.85
NITROGEN
0.1
1
0.2]
1 w
0.4
0.2
MI
P IP O
Figure 5. Adsorption isotherms of neopentane and nitrogen on Carbosieve: open symbols,admrption; closed symbols,deaorption. 0.01
0.1
0.4
P /Po
a
0.8
2.0
1.5
Figure 7. %-Plotsof neopentane and nitrogen for charcoal cloth JF517. 0.01
2.0{
1.0 as
0.5
0.1 ,
a, plots AX21.
I
0.4 ,
P /Po ,
,
,
I
0.
0.8
I
a, plots Charcoal Cloth JF143. NEOPENTANE
__-NITROGEN
- -
NEOPENTANE
4
0.5
1 I
0.5
1-0 as
I-5
2.0
Figure 6. a,-Plots of neopentane and nitrogen for AX21.
pentane molecules through narrow pore entrances.' Examination of selected samples of charcoal cloth by highresolution electron microscopylo has indicated that the former explanation is more likely to be correct. The a,-method used for the analysis of the isotherm was devised as an empirical procedure for characterking porous s o l i d ~ . ~ ~ The ' J ~ a,-plots in Figures 6-10 are constructed (10) Sanders, J. V., private communication. (11) Atkinson, D.; McLeod, A. I.; Sing,K.S.W.J. Chim. Phys. 1984, 81, 791.
1.0 4
-
1.5
"
2.0
Figure 8. cu,-Plots of neopentane and nitrogen for charcoal cloth JF143.
from the isotherms in the manner described previously:'~' the amount adsorbed is plotted against a,,the reduced standard adsorption on nonporous carbon (with a, = 1at p / p o = 0.4). Several nonporous carbon blacks have been used to obtain the standard a, curves for nitrogen12 and ne0~entane.l~ It is apparent that there are some significant differences in the form of the a,-plots. Those in Figures 8-10 reveal a pronounced distortion of the isotherm shape (especially for nitrogen) at very low p / p o ,which can be attributed to (12) Carrott, P. J. M.; Roberts, R. A.; Sing, K. S. W. Carbon, 1987,25, 69.
(13) Roberts, R. A.; Sing, K. S. W., unpublished results.
334 Langmuir, Vol. 3, No. 3, 1987
Roberts et al. Table I. Surface Areas and Pore Volumes of Microporous
0.4-
Carbone nitroeen
-
a, plots Charcoal Cloth JF005.
AB ET^, m2 g-' AX21 JF517 JF143
Carbosieve JF005
neonentane
V ABETp, A?, g-l cm%$l m2 g-' m2 g-' cm g
AsN,
m2
3393 1657 1408 1179 882
233 218 74 41 19
1.53 0.76 0.53 0.44 0.33
3913 1819 924 544 518
130 132 39 17 22
1.70 0.83 0.38 0.24 0.22
'a,(nitrogen) = 0.162 nm2; u(nitrogen) = 0.36 nm; a,(neopentane) = 0.54 nm2; dneopentane) = 0.62 nm.
Table 11. Distribution of Effective Micropore Size total micropore adsorbent vol, cm3 g-' AX21 1.7 JF517 0.83 JF143 0.53 I
J
0.5
1.0
JF005
2.0
1.5
Carbosieve
as
Figure 9. a,-Plota of neopentane and nitrogen for charcoal cloth JF005.
0.01
0.1
0.4
P /PO
0.8
0.85
0.33 0.44
effective pore range, nm distribution 0.62-2 broad 0.6-3+ very broad 0.36-1.2 fairly narrow, -90% < 0.8 nm, -30% < 0.6 nm 0.36-0.8 narrow, -30% < 0.6 nm 0.36-0.7 narrow, -45% < 0.6 nm
the molecular areas as a,(nitrogen) = 0.162 nm2 and a,(neopentane) = 0.54 nm2.14 The "external" areas, AsNand A:, were calculated from the slopes of the a,-plots in multilayer range7 by application of the equations
AsN = 1826V/a,
(1)
A T = 842V/as
(2)
and
0.3 7.n
I
-
,.
, ".
d'
NEOPENTANE
0.5
1.0
1.5
2.0
as
Figure 10. a,-Plots for neopentane and nitrogen for Carbosieve.
the enhanced adsorbent-adsorbate interaction in pores of molecular dimensions, i.e., the process of primary micropore filling.'lJ2 The a,-plots in Figures 6 and 7, on the other hand, are characteristic of microporous adsorbents having a wider range of pore size and giving rise to two separate stages of micropore filling.11*12This is particularly evident in the case of Figure 7 where the upward swing of the a,-plots for both neopentane and nitrogen (at p/p' -0.02) is indicative of the onset of the cooperative process in pores of 2-5 molecular dimen~ions.~Jl Values of surface area and micropore volume derived from the nitrogen and neopentane isotherms are given in Table I. The BET areas, A m N and A s a p , were obtained from the nitrogen and neopentane BET plots, by taking
where the factors 1826 and 842 have been obtained by calibration against the BET nitrogen areas of the nonporous carbon blacks.12 The values of micropore volume, V and Vpp,have been assessed by back extrapolation of t i e linear sections of the c ~ , - p i ~ t s . ~ Inspection of Table I reveals that there is no close agreement between any of the corresponding values of surface area or micropore volume. The differences between the BET and asareas are not surprising in view of the microporous nature of all the adsorbents, and also the lack of agreement between the corresponding nitrogen and neopentane values for JF143, JF0005, and Carbosieve are to be expected in view of their molecular sieve character. The results obtained with AX21 and JF517 are more difficult to understand, but in these cases the differences are probably associated with the wide range of micropore size. The fact that these two adsorbents had a higher affinity for neopentane than for nitrogen (as indicated by the difference in isotherm slope at low p / p o ) is consistent with the difference in molecular size and polarizability of the two adsorptives. No theoretical treatment is at present available that can provide a sound basis for the computation of micropore size distribution from a single is0therm.l If the pores are all of molecular dimensions-as is the case with the molecular sieve zeolites-a range of adsorptive molecules of selected size can be used as molecular probes! but such an approach becomes increasingly difficult to apply as the pore width exceeds -0.6 nm. In our view this difficulty could be overcome and hence molecular probes used to characterize a range of wider micropores, if full advantage could be taken of the different characteristic features associated with the two stages of micropore filling. The (14)McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577.
Langmuir 1987,3, 335-340 following is a preliminary attempt to exploit these principles. In slit-shaped pores, the limiting width for primary micropore filling appears to be little more than 2a, i.e., two molecular diameters.lp3 Thus,the upper pore size limit for primary micropore filling by nitrogen is -0.7-0.8 nm and correspondingly by neopentane is 1.5 nm. The upper limits for secondary micropore filling are uncertain-and must depend to a large extent on the adsorbate-adsorbate interactions-but are probably in the region of 5a, i.e. -2 nm for nitrogen and 3 nm for neopentane. If we accept these limits and assume that primary micropore filling can only occur a t p / p o I0.01, we are able to make a first
-
-
335
semiquantitative estimate of the ranges of pore size present in the various carbons under investigation. In addition, we may assume that the difference between the pore volumes available to nitrogen and neopentane represents the volume contained in pores of effective width
