P. J. REUCROFT, W. H. SIMPSON, AND L, A. JONAS
3526 and Engel16 reported that parent-molecule-plus-CH2 (i.e., “synthesis”) products were obtainable in high yield in the reaction of thermal carbon vapor with hydrocarbons; in their system it was demonstrated that no methylene was involved. To support this mechanism, we have photolytically generated CeH6CH: in benzene solution by the photolysis of styrene oxide. The formation of toluene was firmly established by glpc analysis. We therefore believe that bare C atoms, as well as methylene, are reacting directly with benzene. Another argument that favors the notion that methylene is not the major species involved in the ring labeling of toluene is the following. A methylene route to ring labeling is presumed to involve (after intramolecular hydrogen shifts) such norcaradiene structures as
‘0:. /
5\
4
:
o
3
4
*
g
,
etc.
3
Such structures can only give more C2-labeled toluene than C1-labeled. However, the data of Figure 1 show that C1 is the more highly labeled. These results indicate that a route through methylene is not the principal one in the ring labeling, although it may be for the methyl labeling. It is possible that the relative populations of the various carbon spin states, the ground state lS or the excited ‘D and aP states, may decide the yield and radioactivity distributions of the toluene product. However, the high ring/Me activity ratio in the 5-eV experiment is apparently not the result of the 14C* ion reacting with the benzene before the ion becomes neutralized. Recent results in our laboratory, the subject of a forthcoming publication, have (1) confirmed the 5-eV result and (2) shown that, when the irradiating ion’s kinetic energy is reduced to 3 or 2 eV, the ring/Me activity ratio diminishes to about 0.1. (16) P. S. Skell and R. R. Engel,
J. Amer. Chem. SOC., 88, 4888
(1966).
Sorption Properties of Activated Carbon
by P. J. Reucroft, Department of Material Science and Metallurgical Engineering, University of Kentucky, Lexington, Kentucky 40606
W. H. Simpson, ChemistTy Department, The Franklin Institute Research Laboratories, Philadelphia, Pennsylvania
19105
and L. A. Jonas* Research Laboratories, Edoewood Arsenal, Edgewood Arsenal, Maryland
21010
(Received February 8, 1071)
Publication costs assisted by Edgewood Arsenal
Adsorption-desorption isotherm data were obtained for 15 organic vapors on a BPL grade, PAC, activated carbon. IRotherm data in the form of log W as a function of € 2 for each adsorbate were given to a Univac 1108 computer to determine the characteristic curve equation by means of regression analysis = volume of adsorbed vapor, E = adsorption potential), Equations in the form of the Dubinin-Polanyi equation for fine grained carbon were obtained for each adsorbate. Structural constants for the adsorbent and affinity coefficients for the adsorbates were calculated from the coefficientsof these equations. Affinity coefficients calculated from the experimental data were then compared to theoretical values in order to test the predictive ability of the characteristic curve equation.
(w
Introduction Predictive Isotherm Equations. The ability to predict adsorption isotherms for a given adsorbent from a knowledge of the physical properties of the adsorbate has long been an important objective in solid surfacegas interaction research. Kummer in 1946 studied the relation of Polanyi’s adsorption potential to molecular The Journal of Physical Chemistry, Vol. 76, No. 25, 1971
polarizability. I n 1947, Dubinin and his coworkers first suggested that the following equations could be used for predicting adsorption isotherms2 (1) J. T. Kummer, Ph.D. Thesis, The Johns Hopkins University, Baltimore, Md., 1946. (2) (a) M.M. Dubinin and E. D. Zaverina, Zh. Fiz. Khim., 23, 1129 (1949); (b) M.M. Dubinin, E. D. Zaverina, and L. V. Radushkevich, ibid., 21, 1351 (1947).
3527
SORPTION PROPERTIES OF ACTIVATED CARBON
log
w = log wo -
Equation 1 was suggested for adsorbents of the “first structural type,” that is adsorbents whose pore dimensions are comparable in size to the dimensions of the molecules adsorbed. Equation 2 was suggested for adsorbents with coarse pores, where the pore dimensions are much larger than adsorbate molecule dimensions. I n these equations W is the volume of condensed adsorbate; P is the equilibrium pressure of adsorbate vapor; Po is the saturated vapor pressure of liquid adsorbate a t temperature T’K; W Oand B are constants related to the pore structure of the adsorbent; and /3 is a constant k n o ~ nas the affinity coefficient, which compares the strength of the adsorptive interaction of the adsorbate in question to that of some reference substance. Thus, if @ values are known or can be calculated for several adsorbate vapors, adsorption isotherms can be determined for all the vapors by measuring experimentally the adsorption isotherm of a reference vapor. The problem consists of determining /3 in a general way so that all types of adsorptive interaction are allowed for. The equations can also be written in the following form, e.g., for eq 1 log
w = log wo - k‘e2 - = log wo - ke2 P2
(3)
where e
Po
= RTln-
(4)
P
= gas constant and
k‘
p=--
a!
(7)
aREF
and
R
isotherm. This was done because prior unpublished work on the sorption of dimethyl methylphosphonate (DMMP) vapor by this identical gas phase carbon had shown that more reliable and reproducible data were obtained for the desorption rather than the adsorption isotherm, resulting in a better straight line characteristic curve from the Dubinin-Polanyi equations. This was especially true at low pressures because each time a dose of vapor was desorbed the balance system was cleared of air or lorn boiling impurities. I n the study with DMMP vapor the hysteresis loop at) 25” formed in a narrow band at a PIP0 of 0.65 and disappeared a t about 0.1. The initial adsorption point in the present study was therefore taken at a PIP0 of 0.8 and 0.9 since it was expected to be well beyond the hysteresis range and therefore the adsorption and desorption points should coincide. Experimental @ values so evaluated from measured isotherms can then be compared with theoretical @ values to assess the predictive ability of the isotherm equation. Theoretical Afinity Coeficients for Nonpolar Adsorbates. The main problem in using such equations to predict the isotherm for a new adsorbate vapor is to assign a value of p which correctly describes the nature of the adsorptive interaction. The work of Dubinin and ~ o w o r k e r shas ~ + ~shown that eq 3, 4, and 5 give a good description of adsorption isotherms for systems in which dispersion forces play a dominant role in determining the adsorptive interactions. In this case the adsorptive interaction is strongly dependent on the polarizability (a)of the molecules and @ can be expressed in terms of polarizability.
=
B (2.303)3R2
(5)
Equation 3 is a useful form for comparing adsorption data for several adsorbates on one adsorbent since (1) the equation is not an explicit function of tetnperature ( E is the temperature dependent “adsorption potential” or more rigorously, the change of free energy during the reversible, isothermal transfer of a mole of the adsorbate from bulk liquid to an infinitely large amount of adsorbent) and (2) k depends on the adsorbate parameter @. Thus, if a reference adsorbate is chosen, for which p = 1, experimental p values can be evaluated for other adsorbates from
Since the polarizability of a molecule is approximately proportional to the volume of a molecule or molar volume V, the affinity coefficient can also be expressed asad
p=- V VREF More precisely, the affinity coefficient can be expressed in terms of the molecular parachor $2
where y = surface tension, M and p = density of liquid. Experimental procedure will show that except for the first adsorption point, obtained at a P/Po of 0.8 to 0.9, all subsequent points were obtained from the desorption
=
molecular weight,
(3) (a) M. M. Dubinin, Chem. Rev., 60, 235 (1960); (b) B. P. Bering, M. M. Dubinin, and V. V. Serpinsky, J . CoZZ. Inst. Sci., 21, 378 (1966); ( 0 ) M. M. Dubinin, Izv. Akad. Nauk SSSR Otd. Khim. Nauk, 1153 (1960); (d) M. .M.Dubinin and D. P. Timofeyev, Zh. Fiz. Khim., 22, 113 (1948). The Journal of Phusical Chemistry, Vol. 76, No. 23, 1971
3528
Theoretical A f i n i t y Coeficients for Polav Adsorbates. When the adsorbate molecules have polar nature, i.e., possess a permanent dipole moment, electrostatic interactions may play a greater role in determining the total adsorptive interaction which determines the adsorption isotherm. I n the case of a nonpolar adsorbent there will be contributions from dipole-induced dipole force^.^ If the adsorbent possesses some polar nature due to the presence of ionic impurities, there will be an additional contribution from ion-dipole forces. I n the former case it has been shown that for a dipole moment esu) the electrostatic contribution to of 2 D (2 X the adsorptive interaction in the case of a conducting surface is an order of magnitude smaller than the dispersion interaction.6 An upper limit can be set for iondipole force contributions by considering the interaction between polar molecules and an ionic crystal. In the case of SO2 (dipole moment = 1.6 D) adsorbed on barium fluoride crystals, it has been estimated that the electrostatic interaction is responsible for 5401, of the total interaction, the remainder being accounted for in terms of dispersion forcesS6 If a polar molecule having a dipole moment of 4.0 D is considered, the electrostatic contribution may amount to 70% of the total interaction. Dipole energy terms usually contain the dipole moment as a p2 term. Thus, when electrostatic forces play a dominant role in determining the adsorptive interaction energy it might be expected that p could be expressed as a term in p 2 such as
In intermediate situations, empirical expressions of the type
may be applicable. To extend the Dubinin-Polanyi treatment to adsorption situations where interactions other than dispersion forces are possible, isotherms have been measured for a range of organic vapors on a carbon adsorbent. The general case of nonpolar, weakly polar ( p < 2 D), and strongly polar vapor { p = 2-4 D) adsorbed on a carbon adsorbent reported to have little or no ionic character and having pore dimensions comparable to the adsorbate molecule dimensions, has been investigated. Experimentally determined p values obtained from isotherm data plotted as log W VS. 2 (eq 3) have been compared with theoretical p values obtained fromeq 7-11.
Experimental Section The apparatus used for the measurement of isotherms consisted of two glass vacuum systems. The first system consisted of a train of traps used for vacuum distillation of the organic vapor into a storage vessel. The Journal of Physical Chemistry, Vol. 76, No. 88,1971
P. J. REUCROFT, W. H. SIMPSON, AND L. A. JONAS The vapor was prepurified by distillation to greater than 99% before it was introduced into the vacuum system. A Cahn RG electrobalance and an MKS Baratron’ pressure gauge formed the second system. The carbon sample was suspended from the electrobalance in a small glass container which had been tared with metal weights. A change in mass of the carbon due to adsorption was recorded as an electrical signal from the balance. The pressure changes were recorded as electrical signals due to the change of capacitance between two electrodes in the pressure heads of the Baratron gauge. Some instability was noticed at very low pressures which was attributed to temperature fluctuations in the room. The temperature of the carbon sample was kept constant by a heat sink constructed at FIRL to fit the hangdown tubes of the balance. The system was kept ultraclean by using a General Electric sorption pump for rough pumping and a General Electric Vac-Ion’ pump in series with a Veeco Torgett’ pump for high vacuums. The system was capable of pumping down to 10-6 Torr and maintaining this pressure indefinitely. All stopcocks were Teflon with either silicone or Viton “0” rings. The entire vacuum system was entirely free of oil and grease. The purity of the organic vapors was determined by gas chromatography using a Hewlett-Packard (F & M) 810 chromatograph equipped with dual columns and flame ionization detectors. The columns used for vpc analysis were 10% Carbowax on Chromosorb P (6 ft) and 10% diisodecyl phthalate on Chromosorb P (6 ft). Before each determination the carbon sample was heated to 450” for 8 hr under a vacuum of Torr. When the sample reached a constant mass, the carbon was brought to a constant temperature and the organic vapor was introduced into the system until a P/Po of 0.8-0.9 was attained. The system was allowed to come to equilibrium, and the mass of the material adsorbed and the pressure were recorded. Subsequent points were obtained by reducing the pressure of the adsorbate and recording the decrease in mass of material adsorbed. The carbon adsorbent used was a Pittsburgh activated carbon, Type BPL, 12-30 mesh, which is a commercial adsorbent manufactured from coal for general gas phase application. The total pore volume has been determined to be 1058 m*/g, with 70-7.5% of this volume being attributed to pores less than 20 in diameter. The carbon sample used in the balance was approximately 500 mg. Treatment qf Experimental Data. Isotherm data were obtained for the 15 vapors on the BPL grade acti(4) A. D. Crowell, 1.Chem. Phys., 49, 892 (1968).
(5) D. M. Young and A. D. Crowell, “Physical Adsorptionof Gases,” Butterworths, London, 1962, Chapter 2. (6) V. A. Crawford and F. C. Tompkins, Trans. Faraday SOC.,44, 698 (1948). (7) Registered trademark.
SORPTION PROPERTIES OF ACTIVATED CARBON vated carbon. The desorption data, originally obtained as gram adsorbate per gram carbon for various relative pressures, were converted into cubic centimeters of adsorbate per gram of adsorbent ( W ) for various adsorption potentials. These conversions were made using liquid density values at the test temperatures in accord with the pore volume filling concept of Bering, et ala3b Relative pressures in the form PIPo were converted into adsorption potential ( E ) values by means of eq 4,where e is the potential in calories per mole. The adsorption potential represents the work required of an adsorbent surface to compress a vapor at its pressure P to its maximum pressure Po. Tabulated data in the form of log W as a function of e2 for each adsorbate were given to a Univac 1108 computer for regression analysis. The data were expressed in terms of eq 3, the computer print-out providing least mean square values of log Woand IC. These data are tabulated in Table 1. Using eq 5, values of p for the test adsorbate were calculated, compared to the reference vapors, p = l, for each polarity group. These values are given in Tables I1 and IV. CCh, CHCla, and CH3COCH3were reference vapors for the nonpolar, weakly polar, and polar groups, respectively.
3529
1.0-
-
0.9
r 060
0.7-
A0
.!. 0.60.5 -
O'0.0
'SO 0.4 #)e
0.30.2 -
OJ0
'
1
I
2
4
6
' '
10
8
I2
I
14
I
16
I
18
I
20
I
22
'
24
I
26
:3
Table I : Regression Analysis of Isotherm Data for Organic Vapors Correlation Vapors
CClr Dioxane
2,2,4-TMP Benzene Hexane
CHCla Ethyl acetate Methanol Fluorobenaene Tetrachloroethane Acetone Nitromethane Acetaldehyde Acetonitrile Propionaldehyde
Log WO
-0,37715 -0,29248 - 0.34277 - 0.37202 -0.3425 - 0,35341 -0,37321 - 0.52407 -0.36121 - 0.48506 - 0 37472 - 0.30625 -0.41863 -0,39598 -0.38171 I
le
x
(E)
108
1.9839 3.838 1.3383 2.3821 1.3576 2.588 2.735 16.81 2.026 1.153 3.7685 10.465 4.4128 7.4278 3.9717
0.98880 0.91193 0.76996 0.99602 0.99361 0.99382 0.98270 0 94757 0.98749 0.89737 0 98873 0.99715 0.99183 0.99132 0.99368 a
I
I n addition to the equation of the line, the computer print-out contained the coefficient of which is a measure of the degree with which the data points actually show the particular functional relationship constructed by the regression analysis. As can be seen from the tabulated values in Table I, the coefficient of correlation values were very high, ranging between 0.76996 and 0.99715, indicating excellent agreement between the data points and the computer-derived mathematical equations.
0
Figure 2. Desorption isotherms on activated carbon, 25": 0, carbon tetrachloride; 0,1,1,2,2-tetrachloroethane;X, fluorobenzene; V, chloroform; $3, nitromethane; A, methanol.
Results and Discussion Figure 1 shows a typical plot of log W vs. 2 for the adsorption isotherm data of carbon tetrachloride at 60O.9 Figure 2 shows desorption isotherms at) 25" for six of the vapors tested and indicates the range of values obtained. Of the 15 organic vapors studied, only 2,2,4-trimethylpentane exhibited a large deviation from straight line behavior when the desorption data were plotted in the form of Dubinin-Polanyi equation. This is obvious from the value of the correlation coefficient ( R ) for 2,2,44rimethylpentane listed in Table I. (8) P. G. Hoel, "Introduction to Mathematical Statistics," Wiley, New York, N. Y.,1954, p 120; M.R. Spiegel, "Statistics," Schaum Publishing Co., New York, N. Y., 1961, p 224. (9) A comprehensive tabulation of all isotherm data as log W V S . ea may be obtained from the Final Report Contract No. D A A A 15-69C-0712.
The Journal
of
Physical Chemistry, Vol. 76,No. $3,1071
P. J. REUCROFT, W. H. SIMPBON, AND L. A. JONAS
3530 Table I1 : p Values Calculated Using Electronic Polarization
-___ 8, experimental Groups
Nonpolar
Weakly polar O
