Langmuir 1987, 3, 708-713
708
Adsorption Capacity of Molecular Sieve Type Carbons? A. N. Ainscough Department of Chemistry, University of Salford, Salford, England
David Dollimore* Department of Chemistry, University of Toledo, Toledo, Ohio 43606 Received October 24, 1986. In Final Form: March 10, 1987 The preparation of carbons from polymeric sources, such as ethylcellulose, poly(acrylonitrile),cellulose, poly(vinylidene chloride), and poly(furfury1alcohol), is described. All these carbons showed varying degrees of adsorptive capacity. A preparation of a composite carbon based on a commercial active carbon is described. Viscose carbons reported in an earlier publication are studied further. The adsorption isotherms on these carbons for the following adsorbates are reported (K): nitrogen (77.4), carbon dioxide (194.7), carbon monoxide (77), water (298.2),nitrous oxide (194.7),argon (77.4), methane (90.2), ethane (194.7),propane (194.7),n-butane (273.2),cyclopropane (194.7),carbon tetrachloride (298.2),benzene (298.2),a-pinene (298.2), and neopentane (273.2). These are mainly type I adsorption isotherms. The apparent pore volume as determined from the uptake of these adsorbates at PIPo = 0.95 shows that some of these carbons showed selectivity based on adsorbate molecular size while others, while still showing large apparent pore volumes, showed no selectivity of this kind. These isotherms are analyzed by methods based on the BET and Dubinin theories. In particular, a relationship between the BET C value and the Dubinin D value is noted, based on the fact that the adsorption isotherms are amenable to analyses by both methods. It is concluded that the following parameters contribute to the selectivity that carbons show to adsorbate capacity, namely: (1)adsorbate molecular size, (2) temperature of the adsorption isotherm, (3) shape of the adsorbate molecules, (4) prior adsorption of water vapor, and (5) shape of the pore openings and general pore structure. The evidence indicates that there exists a wide distribution in slit width, but the argument that slit width can be computed from the dimensions of the molecules it adsorbs and those that it excludes would appear to be refuted by the evidence presented in this study.
Introduction In recent publications the preparation and properties of nongraphic carbons has been described.'r2 The development of microporosity in some carbons has been The structural analysis based on the adsorption isotherms on these materials has been supported by the Dubinin-Radushkevich t h e o e and by the more general theory of Dubinin-Astakho~,~Another method of investigating microporosity has been by adsorption isotherm subtraction and nonane preadsorption.lOJ1 Most of these publications also refer to the BET method to provide a so-called monolayer capacity.12 In the case of type I adsorption isotherms it is probably better to regard this as an adsorption capacity at a characteristic point on the isotherm rather than proceed to the calculation of the specific surface area. These comments and method of analysis also apply to fiber carbons.13,14 The DubininRaduchkevich (DR) equation is log V = log Vo - D log2 (Po/P)
V 1 c-1P =-++(3) V(P0- P ) v,c V,C Po where C is the BET constant. The significance of the same data fitting both the BET equation and the Dubinin equation has been analytically examined13 and can be expressed in the form V,/Vo = exp[-D ln2 (P0/P),] = exp[-D ln2 ( ( C - l)/(-l + C1/'))] (4) There are reports on the ratio V,/Vo inferring that the ratio V,/ Voshould by unity.15J6 The value of Voshould be larger than V , because V,, represents an extrapolation to saturated vapor pressure. The inference that V,/ Vo should be unity is illusory, for it represents a limiting case. Thus when V , = Vo,eq 4 reduces to 1 = exp[-D ln2 (Po/&] when (P/Po), = 1 and the BET monolayer V , occurs at saturation. This will only occur with the perfect Langmuir
(1) (1) Rand, B.; McEnaney, B. Br. Ceram. Trans. J. 1985, 84, 157.
where V is the volume of the adsorbate adsorbed at relative pressure PIP,, Vois the micropore volume, and D is the DR isotherm constant. The Dubinin-Astakhov representation is log V = log Vo - D' log" (Po/P)
(2)
m is usually given a value of 2 or 3, but curve fitting will result in noninteger values. In these representations it is reasonably easy in the case of any microporous adsorbent to determine the value of Vofor any adsorbate by direct inspection of the adsorption isotherm. The V , value (the monolayer capacity) can be calculated from the BET equation:12 '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.
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McEnaney, B.; Rand, B. Br. Ceram. Trans. J. 1985,84, 193. (3) Masters, K. J.; McEnaney,'B. Carbon 1984,22, 595. (4) Ainscough, A. N.; Dollimore, D.; Heal, G. R. Carbon, 1973,11, 189. (5) Culver, P. V.; Heath, N. S. Trans. Faraday SOC.1955, 51, 1569. (6) Masters, K. J.; McEnaney, B. J. Colloid Interface Sci. 1983, 95, (2)
340. (7) Dubinin, M. M.; Zaverina, E. D.; Radushkevich, L. V. Zn. Fiz. Khim. 1947,21, 1351. ( 8 ) Dubning, M. M. In Chemistry and Physics of Carbon; Walker, P. O., Jr., Ed.; Arnold: London, 1966; Vol. 2, p 51. (9) Dubinin, M. M.; Astakhov, V. A. Adu. Chem. Ser. 1970, 102, 69. (10) Gregg, S. J.; Langford, J. F. Trans. Faraday SOC.1969,65,1394. (11)Ali, S.; McEnaney, B. J. Colloid Interface Sci. 1985, 107, 355. (12) Brunauer, S.; Emmett, P. H.; Teller, E. J.Am. Chem. SOC.1938, 60, 309. (13) Ainscough, A. N.; Dollimore, D.; Spooner, P. In Adsorption at the Gas-Solid and Liquid-Solid Interface; Rouquerol, J., Sing, K. S. W., Eds.; Elsevier: Amsterdam, 1982; p 187. (14) Hitchcock, S. J.; McEnaney, B.; Watling, S. J. J. Chem. Technol. Biotechnol., Chem. Technol. 1983,33A, 157. (15) Klemperer, D. F. In Surface Area Determination; Everett, D. H.; Ottewill, R. H., Eds.; Butterworths: London, 1970; p 59. (16) Gottwald, B. A. In Surface Area Determination; Everett, D. H.; Ottewill, R. H., Eds.; Butterworths: London, 1970; p 59. 0 1987 American Chemical Society
Langmuir, Vol. 3, No. 5, 1987 709
Adsorption Capacity of Molecular Sieve Carbons VOLUME
ADSOR BEC NOTED AS LIQUID
VOLUME PER GRAM
MOLECULAR DIAMETER
OF ADSORBATE
Figure 1. Schematic representation of the adsorption of different molecular sized adsorbates onto a microporous solid. The plot shows that the majority of the pores will adsorb molecules with a value less than X.
type adsorption isotherm when the number of monolayers is restricted to one. It has been found for microporous carbons that the ratio V,/ Vo V0.J V0, (where Vo.l is the volume of adsorbate taken up at PIPo = 0.1 and Vo,9is the volume taken up at PIP, = 0.9). This was illustrated for fiber carbons13and holds for a large number of systems showing microporous behavior. However, on Saran charcoals t h e larger adsorbate molecules were not adsorbed to the same extent as smaller adsorbate molecules. If plots of the volume uptake representing t h e microporous volume are plotted against some measure of size of the adsorbate molecule for an adsorption isotherm of different adsorbates on a given microporous carbon, then an exclusion principle can be demonstrated for microporous materials. This amounts to noting t h e volume uptake at PIPo = 0.8-0.9 or o n t h e "plateau" of the adsorption isotherm. This has been demonstrated for certain silicas and for ~arbons.~J'Plots of the kind shown schematically in Figure 1can be constructed. This method is applied in t h i s study. Various carbons were prepared and their adsorption capacity for different sized adsorbate molecules measured.
Experimental Section Carbons. The carbons selected were taken from as wide a range as was immediately possible. Those included are an ethylcellulose carbon, a poly(acrylonitri1e) carbon, a cellulose carbon, a poly(viny1idene chloride) carbon, commercially available active carbons, and a selection of fiber carbons. The preparation and certain adsorption characteristics of the poly(viny1idene chloride) carbon were previously reported! Nitrogen adsorption characteristics of the poly(viny1idene chloride) carbon fibers can be found in another study.13 Table I indicates the sources of the various carbons. The poly(vinylidenechloride) preparation is described elsewhere! The poly(acrylonitri1e) was prepared by polymerization of acrylonitrile in aqueous solution.'* Carbonization of the raw materials was carried out in a flowing atmosphere of 99.99% dry nitrogen in open crucibles. Activation was performed in the same equipment with carbon dioxide. Details have been previously reported.I3 The composite carbons were prepared by using the commercial active carbon listed in Table I. It was coated with furfuryl alcohol, recarbonized, and activated. Two separate samples of the composite carbons were prepared, one being polymerized a t room temperature and the other at 90 "C. Some 10 g of the active carbon was mixed with 100 mL of furfuryl alcohol containing 0.4% (17) Dollimore, D.; Heal, G. R. Trans. Faraday SOC.1963,59, 2386. (18) Elliot, J. R. Macromol. Synth. 1966, 2, 78.
Table I. Sources of Selected Carbon Adsorbents carbon source . reason for choice ethylcellulose Hercules Powder Co. low carbon content poly(acrylonitri1e) prepared in the lab polymer containing nitrogen cellulose avicel-microcrystalcrystalline polymer line cellulose (Honeywell and Stein) poly(viny1idene ref 4 no tar evolved in the chloride) carbonization process active carbons base carbon source for composite carbon preparation viscouse fiber Le Carbonne high-capacity carbons carbon preparation and lab retaining fiber preparation from characteristics viscouse fibers (ref 13) poly(furfury1 lab preparation used for composite alcohol) carbon preparation Table 11. Adsorption Isotherm Details density of temperature of adsorbed Po, adsorbate of isotherm, K liquid, g mL-' cmHg nitrogen 77.4 0.808 76.0 carbon dioxide 194.7 1.260 141.4 carbon monoxide 77 0.817 46.72 water 298.2 0.997 2.3756 nitrous oxide 194.7 1.201 137.0 argon 77.4 1.427 22.36 methane 90.2 0.457 8.435 ethane 194.7 0.535 128.5 propane 194.7 0.630 10.66 n-butane 273.2 0.600 78.5 cyclopropane 194.7 0.720 6.033 carbon tetrachloride 298.2 1.549 11.319 benzene 298.1 0.879 9.523 a-pinene 298.2 0.850 0.477 neopentane 273.2 0.613 53.50
of 2 N hydrochloric acid and refluxed under nitrogen for 5 h at 90 "C (the other sample was allowed to stand at room temperature for 5 h). The excess liquid was then decanted off, and the composite carbons stood under nitrogen for 4 days. Carbonization was then carried out as noted above. Adsorption Measurements. Adsorption measurements were made with nitrogen at -196 "C by using a volumetric unit.lg A Cahn balance was used for the gravimetric measurement of the adsorption isotherms. The mass of carbon used in these experiments was 0.4 g. The carbons were degassed at 200 "C in vacuum for 12 h. The adsorbates were purified by distillation." The carbon tetrachloride was a spectroscopic grade and stood over anhydrous sodium sulfate for 1week. The benzene and a-pinene were dried over sodium. Table I1 lists the adsorption isotherm details.
Results and Discussion The carbons prepared and studied were investigated to determine their molecular sieve properties. Molecular sieve materials are of importance in industry where their selectivity is utilized.20 Zeolites owe their selectivity to restricted pore openings. Carbons c a n be prepared with large adsorptive capacity and no selectivity, or t h e y can be prepared with molecular sieve properties and pores supposedly ranging from 12 A.21,22 Walker e t ala produced (19) Gregg, S. J.; Sing, K. S.W. J.Phys. Chem. 1951, 55, 592. (20) Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T. L. J.Am. Chem. SOC.1956, 68, 5963. (21) Wolff, W. F. J. Phys. Chem. 1958, 62, 829. (22) Kipling, J. J.; Wilson, R. B. Trans. Faraday SOC.1960, 56, 557. (23) Walker, P. L.; Lamond, T. C.; Metcalfe, J. B. 2nd Conference on Industrial Carbon and Graphite; Society of Chemical Industry: London, 1966; p 7.
710 Langmuir, Vol. 3, No. 5, 1987
Ainscough and Dollimore
200
5
250-
150
200-
; 7
yc 2
I00
D
ETHYL
CELLULOSE
0
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CARBON
50
01
01
0 2
0 3
04 RELATIVE
0 5
0 6
PRESSURE
0 7
08
09
10
Ii
ypo
Figure 2. Adsorption isotherm of nitrogen on microcrystalline cellulose carbon. 250
4
0
0.2
0.1
200-
,
/
0.4
0.3
RELATIVE
PRESSURE
Figure 4. Adsorption isotherms of nitrous oxide on two carbons and a molecular sieve.
I
01
0 2
03
04
RELATIVE
0 5
0 7
0.6
0 8
0 9
1.0
%.
PRESSURE
Figure 5. Adsorption isotherm of butane on composite carbon (room temperature preparation). BEFORE
e2
WATER
ADSORPTION
500
y
,
2 p , 9 400-
AFTER
WATER
ADSORPTION
300-
*ooj
lool( 1
0
01
0 2
0 3 WELATIVE
0 4
0 5
0 6
07
0 0
0 9
10
PRESSURE
Figure 6. Adsorption of argon on the base carbon before and after water adsorption.
(24) Pauling, L. In The Nuture of the Chemical Bond; Cornell University Press: Ithaca, NY, 1945, p 189. (25) Huggins, M. L. J. A m . Chem. SOC.1953, 75, 4126. (26) Foster, A. G., Discuss. Faraday SOC. 1948, 3, 41. (27) Emmett, P. H. Chem. Rev. 1948, 43, 69. (28) Barrer, T. M. Annu. Rep. Pros. Chem. 1944, 41, 32. (29) Bond, R. L.; Spencer, D. H. T. Industrial Carbon and Graphite; Society of Chemical Industry: London, 1958; p 231. (30) de Boer, J. H. In The Dynamical Character of Adsorption; Clarendon: Oxford, 1953; p 146.
uptake only near PIPo = 1. One notable point here is that the large adsorbate molecules often also gave type I adsorption isotherms extending usually from ca. PIP, = 0.1 to PIPo = 0.98. A typical nitrogen adsorption isotherm is shown in Figure 2. The adsorption of carbon dioxide by ethylcellulose carbon is given in Figure 3. Most of the carbon dioxide isotherms reached a level plateau of adsorption by PIPo = 0.3 or 0.4. The adsorption of nitrous oxide is portrayed in Figure 4 on two carbons, and it can be seen that the character of the adsorption is similar to the adsorption of the same gas on a 5A molecular sieve. These serve as examples, and in this study the adsorption capacity a t PIP, = 0.9-0.95 is noted for comparison
Adsorption Capacity of Molecular Sieve Carbons Table 111. Molecular Size of Adsorbates" molecular adsorbate diameter, A method of estimation 3.00 2x covalent radius of N atom nitrogen 2 X covalent radius of 0 atom carbon dioxide 2.80 2X covalent radius of 0 atom carbon monoxide 2.80 water 3.15 ref 20 2 X covalent radius of N atom nitrous oxide 3.00 ref 20 3.84 argon 2 X radius of CH3 group methane 4.00 ref 20 propane 4.89 2 X radius of CH3 group 4.00 ref 20 butane 4.89 4.00 2X radius of CH3 group max diameter from a molecular cyclopropane 5.78 model 2 X radius of CH3 group 4.00 diameter from a molecular model 6.00 carbon tetrachloride (CC13F) ref 20 6.90 2 X thickness (ref 23) 3.70 benzene ref 20 6.8 total height from a molecular neopentane 6.07 model diameter from a molecular model 7.08 4-membered ring strained to a-pinene 6.92 120" to the vertical 8.01 4-membered ring strained to 180° to the vertical ~~
Langmuir, Vol. 3, No. 5, 1987 711
:I
O 0.4 'I/
01
a PINENE
0
1.0
30
20
MOLECULAR
50
40
60
70
80
DIAMETEP/;
Figure 7. Graph of the pore volume against molecular diameter of adsorbate for a composite carbon (room temperature preparation). 0.3
0.2
0.1
0 10
2 0
3 0
50
4 0
MOLECULAR
DIAMETER
6 0
70
(8)
"The values used to establish molecular sizes from models are taken from Paulingz4and HugginsaZ5
Figure 8. Graph of the pore volume against molecular diameter
purposes. However, two final examples are chosen, namely, the adsorption of butane on the room temperature preparation composite carbon. The butane adsorption showed the usual initial adsorption at low relative pressures followed by a plateau of no further adsorption up to relative pressures higher than 0.95 (see Figure 5). The adsorption of argon shown in Figure 6 shows another problem which arose in that the adsorption capacity was considerably reduced by prior adsorption of water. In the present study only one case of this kind is reported, and further studies on this phenomenon are in progress. The uptake of adsorption isotherms for each carbon adsorbate (taken from the volume of adsorbate considered as a liquid) taken at ca. PIP, = 0.95 was plotted against the molecular size of the adsorbates. In previous work this approach has been adopted to demonstrate microporosity in silica." The diameters of the adsorbates were calculated from bond angles and atomic radii.22 The data are given in Table I11 and have been used in the work on Saran charcoal? The directional variation of the size of adsorbate is clear from this table. Plots can be made, however, by using this data. Figure 7 shows data of this kind showing
a selectivity in terms of size of the adsorbate molecules. Figure 8 is a similar plot of adsorption capacities for various adsorbates on the microcrystalline cellulose carbon. However, there are numerous methods of calculating molecular size, and they all tend to give different values. This is demonstrated in Table IV. The list is not exhaustive, and a rigorous search would allow the table to be completed. The adsorption capacities of the other carbons are therefore portrayed here by listing the adsorbates in order of molecular size, as noted in Table 111, and by tabulating the information. The adsorption capacities for these carbons and other similar adsorbents, such as molecular sieve 5A and a silica molecular sieve, are noted in Tables V and VI, together with data showing the percent of available pores filled at PIP, = 0.1. It is seen that in many of the carbons there is in operation a selectivity principle that appears to be in the rough order of adsorbate molecular size. Other carbons, however, retain a high adsorptive capacity whilst retaining type I adsorption isotherm characteristics for all adsorbates listed, regardless of the molecular size of the adsorbate. Reference to Table 11, however, points to a possible weakness in making too
for the microcrystalline cellulose carbon.
Table IV. Molecular Diameters (A) of Adsorbate Molecules Determined by Various Methods absorbate a b C d e f g h 2 nitrogen 4.33 3.00 4.08 3.40 3.15 3.00 3.15 3.15 argon 4.09 3.84 3.84 2.94 2.95 3.84 2.88 2.94 water 3.48 3.10 2.76 3.3 2.89 3.15 2.89 nitrous oxide 3.27 3.00 n-butane 6.20 4.89 7.78 4.60 4.0 4.51 3.7 benzene 5.92 6.80 3.7 or 7.0 carbon tetrachloride 6.09 6.gk 6.1 4.79 6.0 methane 4.57 3.24 4.00 3.24 ethane 4.00 propane 5.22 4.89 4.0 neopentane 6.92-8.01 a p in ene 6.92-8.01 carbon dioxide 2.8 3.34 3.23
i 3.53
-
3.40
From liquid density according to Fosterz6 and Emmettn bBreck et aLZ0cBarrer (length of chain or diameter).2s dBond and Spencer.% 'Kipling and Wilson.22 fFrom van der Waals constant according to de Boer.30 #From molecular models.' hFrom viscosity. 'From van der Waals constant. jFrom heat conduction. kBased on CCI3F. a
712 Langmuir, Vol. 3, No. 5, 1987
Ainscough and Dollimore
Table V. Adsorption Capacity (mL g-') at P / P o = 0.95 and Percent of Pores Filled at P / P o = 0.1 for Various Adsorbates on Carbons and Molecular Sieves Sutcliffe carbon zeolite molecular (after water poly(acry1onitrile) molecular sieve 5A sieve silican Sutcliffe, carbon adsorption) carbon % pores adsn % pores adsn % pores adsn % pores adsn % pores adsn adsorbate caoacitv filled caoacitv filled caoacitv filled caoacitv filled caoacitv filled nitrogen 0.097 91.91 0.506 88.8 0 0.167 95.3 0.539 96.5 0.018 81.8 carbon dioxide 0.490 90.0 carbon monoxide 0.085 0.459 water 64.8 0.197 68.3 0.490 94.0 94.1 0.100 nitrous oxide 84.5 0.315 91.1 0.081 94.7 0.494 argon 0.236 92.6 0.438 88.1 methane 71.7 91.2 0.504 0.187 ethane 70.4 0.513 91.3 0.175 propane 0.167 75.5 0.073 68.8 0.490 79.7 butane 76.5 0.046 cyclopropane 0.022 0.017 40.0 0.4 90.0 0.326 83.6 0.004 carbon tetrachloride 69.2 0.495 98.2 0.356 88.8 0.006 5.0 0.017 0.040 benzene 0.0152 0.493 97.0 0.282 82.5 0.005 a-pinene See ref 17. Krypton adsorption capacity 0.069 mL g-'; ethylene adsorption capacity 0.094 mL g-'.
Table VI. Adsorption Capacity (mL g-*) at P / P o = 0.95 and Percent of Pores Filled Carbons poly(viny1idene ethylcellulose composite carbon chloride) carbon" carbon (90 "C prep) adsn % pores adsn % pores adsn % pores capacity filled adsorbate capacity filled capacity filled nitrogen 0.619 83.0 0.158 84.1 0.153 90.2 91.6 0.151 79.8 0.151 91.5 carbon dioxide 0.469 carbon monoxide 0.179 94.9 90.2 0.167 91.0 0.208 84.0 nitrous oxide 0.420 0.420 94.6 argon 97.1 0.114 80.0 0.155 97.1 0.525 methane 0.122 0.191 90.0 0.607 92.3 81.5 ethane 93.5 71.4 0.165 96.2 0.521 0.064 propane 0.158 67.9 0.392 85.5 0.133 69.5 butane 94.1 0.417 cyclopropane 0.016 82.9 0.414 95.5 0.070 carbon tetrachloride 0.091 45.3 0.114 68.0 98.3 0.365 benzene 0.038 92.5 0.392 0.020 neopentane 69.2 0.009 75.0 82.9 0.070 0.301 a-pinene (I
at P / P o = 0.1 for Various Absorbates on composite carbon (room temp prep) adsn % pores capacity filled
microcrystalline cellulose carbon adsn % pores capacity filled
0.261 0.212
83.4 69.7
0.167
93.6
0.279
80.6
0.187
99.0
0.300 0.293 0.270 0.300
92.0 87.9 87.1 91.1
0.236 0.219
87.0 91.6
0.175
99.0
0.000 0.159 0.139 0.008
47.1 94.1
0.004 0.102 0.065 0.019
46.3 44.0 46.2
Data for poly(viny1idene chloride) carbon taken from ref 4.
detailed an analysis from these results. This table clearly shows that the adsorption isotherms were determined at various temperatures, so some of these differences in adsorption capacity and selectivity must be related to these temperature differences. The length of the hydrocarbon chain has been shown to be a real factor in determining apparent adsorption capacity,lOJgand selectivity must be related to these temperature differences. There are some indications that this could be occurring in this study. If the selectivity shown in Figures 7 and 8 and Tables V and VI, was compounded solely of size factors, then one would have expected drastic variations in the type of adsorption isotherm for the larger molecular size adsorbates which were excluded from micropores. This occurred for the microporous silica," where the larger molecular sized adsorbates did produce different types of adsorption isotherm. However, here the carbons showed no drastic change in adsorption isotherm character for the larger molecular sized adsorbates except in a few instances. Even in the Sutcliffe carbon where no real selectivity could be discerned (Table V) all the adsorption isotherms remained Type I in character. In Table VI1 the calculation of the specific surface area is noted from linear BET plots (eq 3). Data of this kind have been previously produced for the polyvinylidene ~ a r b o n .The ~ linear data of both the BET plots and the Langmuir plots have already been noted. The intercepts in both types of plot were zero or
Table VIJ. Nitrogen-Specific Surface Areas and D Values for Various Carbons nitrogen-specific area, m2 g-' 1278 Sutcliffe carbon poly(viny1idene chloride) carbon 1260 ethylcellulose carbon 487 composite carbon 90 "C 501 822 room temp microcrystalline cellulose carbon 701 various carbon fiber samples 1
2 3 4 5 6 7 8 9
molecular sieve silica
387.6 58.5 83.9 114.9 1.82 328.6 36.4 100.0 73.3 250
Dubinin Dubinin D value -0.096 -0.088 -0.076 -0.066 -0.064 -0.110 -0.098 -0.088 -0.039 -0.292 -0.111
-0.309 -0.0432 -0.105
negligible. The linear range of the BET plots was less than normal. In microporous materials the significance of this calculation is that it provides a characteristic adsorption capacity-atlow relative pressure. This can, however, easily be done by comparing the amount adsorbed at PIPo = 0.1 with the amount adsorbed at PIP, = 0.90-0.95. This is
Langmuir 1987,3, 713-721 essentially the data provided in Tables V and VI. In this study the comments on the Dubinin equation are restricted to noting that where the BET equation and the Dubinin equation fit the curve for the adsorption isotherm there is a correlation between the BET parameters and the D value as noted in eq 4. If the BET parameters are known, then the D values can be calculated. This relationship has previously been dem0n~trated.l~ The values of D recorded in Table VI1 have been calculated in this manner. The values of D generally lie between -0.04 and -0.11, which might be a reflection of the porous structure in carbons. Inspection of eq 4 shows that the constant D is compounded of BET values which represent specific amounts adsorbed and the BET constant C (in theory related to the heat of adsorption).
Conclusions One must regard the following parameters as contributing to the selectivity that carbons show to adsorbate capacity, namely: (1)adsorbate molecular size, (2) tem-
713
perature of the adsorption isotherm, (3) shape of the adsorbate molecules, (4) prior adsorption of water vapor, and ( 5 ) shape of the pore openings and general pore structure. The evidence indicates that there exists a wide distribution in slit width, but the argument that slit width can be computed from the dimensions of the molecules it adsorbs and those that it excludes would appear to be refuted by the evidence presented in Tables V and VI and in Figures 7 and 8. There is no change in the character of the adsorption isotherm when a selectivity process excludes adsorbate molecules from smaller pores. Even when no selectivity principle operates, the adsorption isotherms are still type I. It would also seem feasible that the cluster of Dubinin D values is also associated with particular energy site distribution characterizing the pore surface of carbon adsorbents. The use of different sizes of adsorbate molecules to study microporous solids is seen to be helpful both practically and for theoretical reasons. Registry No. Carbon, 7440-44-0.
Chemical and Geometric Factors in Physical Adsorption/Desorption of Gases on Solidst E. L. Fuller, Jr.,* and K. A. Thompson Plant Laboratory, Oak Ridge Y-12 Plant,t Martin Marietta Energy Systems, Inc., Oak Ridge, Tennessee 37831 Received November 24, 1986. In Final Form: April 28, 1987 Physical adsorption of gases affords the most informative means of evaluation of the specific surface area of powdered and porous materials in many areas of technology. Original approximationsassuming that the substrate is inert and that multilayer formation is independent of the chemical composition of the substrate are shown to be invalid. The autoshielding potential treatment of sorption data provides two adjustable parameters that characterize nonporous oxides with respect to energetics and capacity. Deviations from this reference rectilinear behavior serve to delineate the nature of the micropores and mesopores both within and between powder particles. Studies on flat surfaces and microporous material show that the energetics of adsorption are controlled by both the chemical structure and the microporosity. Comparison with classical methods of data analyses shows the versatility of this thermochemical method and the misinterpretations often accompanying the erroneous classical models. The use of high quality and extensive data is emphasized to allow the investigator to deduce relevant information regarding both the size and shape of pores. Results are given for beryllium oxide, aluminum oxide, silicon dioxide, and uranium dioxide materials of interest in commerce and industry.
Introduction Vapor sorption studies are the most informative means of defining the chemistry, morphology, and topology of finely divided materials. Considerable progress has been made from the first recognition that surfaces attract and hold sorbate molecules at chemical potentials greater than that of the corresponding liquid and/or solid phases.l Adsorption isotherms have been recognized to contain valuable information related to the amount and nature of the surface and porosity of the substrate for some time; the entire field of colloid and surface chemistry has risen to academic, industrial, and commercial importance predicated to a significant degree upon the results of such 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. f Operated for the U.S. Department of Energy by Martin Marietta Energy Systems, Inc., under Contract DE-AC05-840R21400.
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experiments. Several inorganic oxide materials have been studied, and the data have been analyzed by different means to evaluate the contribution of the various pertinent factors to the sorption processes. Specifically, sorption processes are strongly influenced by the chemical nature of the sorbate and the sorbent as well as by the pore size and pore shape of the sorbent.24 In this report, we shall systematically consider the contributions of each of the above factors to the overall picture. Experimental Equipment and Materials Data were obtained by using an automated,computer-controlled volumetric sorption apparatus (Micromeritics Model 2500) with (1)Polyani, M. Verh. Dtsch. Phys. Ges. 1914, 16, 1012. (2) Steele, W. A. The Interaction of Gases With Solid Surfaces, Pergamon: Oxford, 1974. (3) Greg, S. J.; Sing, K. S.W. Adsorption, Surface Area, and Porosity, Academic: New York, 1967. (4) Sing, K. S. W. et. al. Pure Appl. Chem. 1985, 57, 603-619.
0 1987 American Chemical Society
