Jonathon C. Hanron and Fred E. Stafford
Northwestern University Evanston, Minois
1
I
Adsorption of Benzene on Silica Gel A high vacuum experiment
A
study of adsorption gives insight into the activity, energy, and physicochemical structure of the surface of the adsorbent. Such studies are of interest both from the point of view of catalysis, and also from that of space age materials and lubricants. The purpose of this experiment is to measure the adsorption isotherm of benzene on silica gel and to relate the isotherm to theory by which the surface area of the silica gel sample is calculated. It is based on an experimental method of A. V. Kiselev (1). The method is both unique and simple because measurements are made a t room temperature using a calibrated capillary to measure the amount of liquid adsorbate transferred. Benzene is chosen as the adsorbate because the force of interaction is believed to be between the ?r electrons and the silanol groups on the gel surface. However, other liquids (%'el with adequate room temperature vapor pressure can be used to minimize the need for a grease-free (no stopcocks) vacuum apparatus. Other methods and discussion are given by Shoemaker and Garland (3) and by Joy (4).
Po and PI are the initial and final pressure in the dead volume in Torr, Pais the ambient atmospheric pressure, and K is one-half the volume of the manometer tube per cm. The silica gel sample was then heated a t 150" under vacuum to drive off any surface impurities (alternatively, it could be heated to approximately 450°C to destroy the =Si-OH group). Pure benzene was placed in the sample bulb SB and degassed several times by freezing it with a dry ice-chloroform-carbon tetrachloride slush. It was then heated with an IR lamp while an ice-water slush (if necessary, with added salt) was placed around the capillary. The ice-water slush was used intcrrriittently so that the benzene in the capillary never froze, because bubbles (which are extremely difficult to remove) form upon thawing. When the benzene is just above freezing it becomes cloudy, making it easy to determine when to remove the icewater bath. When the capillary was filled, both the capillary and the silica gel sample bnlbwere thermostated with water
The Experiment
Figure 1 is a drawing of the vacuum system constructed so that no stopcock grease or O-ring is allowed to come in prolonged contact with the benzene vapor. Four types of gauges were used to measure pressures. For the actual measurement of the adsorption isotherm, an ordinary manometer with cathetometer was used. To illustrate low pressure measurements, a thermocouple gauge, a Philips ionization gauge, and a McLeod gauge were used. With the mercury diffusion pump, pressures of mm Hg were obtained (3, 6,6). Grace-Davison silica gel' was used. The sample was soaked iq distilled water overnight to restore any hydroxyl groups that might have been driven off. It was then dried in a 120" oven for several days. Finally, about 0.30 gm of the sample was weighed out on an analytical balance and placed in a sample bulb which was then sealed onto the vacuum system and checked for leaks. The dead volume Vd between valves 3, 6, 7 and the mercury level a t zero pressure was measured, using dried air from the atmosphere and the known standard volume. From conservation of mass and the gas law where Vd and V. are the dead and standard volumes, 1 W. R. Grace Co., Davison Chemical Division, Baltimore 3, Md. Manufacturer's specification: specific area, 60&7Q0 rn'/gn; pore volume, 0.4 cc/grn; avg. pore diameter, 32A; mesh siae, 60-120; purity, 99.85%.
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Figure 1. The vacuum iine-pump is a ?-stage mercury diffudon pump; 1, 4, 5, 6, and 7 are Fircher-Porter Teflon needle valves; 2 and 3 ore HOKE 513F stoinlers steel valve,; 8, 9 and 10 ore ground glass stopcocks; G l , G2, G 3 ore thermocouple, Phillip, and Mcleod gauger; CAP. Trvbare capillary 0.02 cclcm by 30 cm long; SGS, silica gel romple bulb; M, mercury manometer with fritted glass disc, FG, to retain the mercury; SB, benzene sample; and VOL is standard volume, about 12 cc.
2 or 3" below room temperature in Dewar vessels. (Variation of temperature in either can be significant.) All valves were closed and the pumps shut down. The outlime of the actual measurement of the adsorption isotherm of benzene on the silica gel sample is as follows: (1) Measure the initial benzene height in the capillary and the
pressure, using cathetometers. (2) Admit a small amount of benzene vapor tbrough Valve 3 into the dead volume. The wlve should be kept warm with an IR lamp to prevent benzene from condensing in it. (3) When equilibrium is established, record the height of hensene, the pressure, and various temperatures. (4) Repeat steps (2) and (3). Each data. point takes from 5-15 min to record.
Data
The amount of benzene transferred into the dead volume can be calculated from the decrease of volume in the capillary and the density, d, of benzene. The total number of moles of vapor in the dead volun~eafter adsorption is calculated from the ideal gas law. From conservation of mass, the number of moles adsorbed is obtained by difference, hence the reason for making the dead volume as small as convenient. The data are shown as a plot of the number of moles adsorbed, n,, versus the pressure in Figure 2. Other isotherms for benzene are shown in Refs. (8). There may be a number of sources of error in these data. Although both the capillary and the sample bulb were thermostated with water 2 or 3 T below room temperature, the temperature dependence of the calculations is significant. Assuming the ClausiusClapeyrou equation to be linear over a short range, we write AlnP E
$AT
For AH (ads) = AH (vap) = 10,000 cal/mole, a significant change results from a one degree change in temperature. Some evidence of the temperature stability and reproducibility is given by the overlap of the ascending and descending iostherms in Figure 2. The equilibrium pressure for the isotherm a t saturation should equal the vapor pressure of benzene, 8.88 cm Hg (7). Figure 2 shows reasonable agreement. Furthermore, when the silica gel bulb was cooled with dry ice-acetone, the manometer pressure fell to zero, indicating the absence of inert gas due to leaka
Even in their most general form, these equations fail to describe an observed reversible hysteresis. This phenomenon, not generally discussed, is of importance here because it may occur for some silica gel samples @a). By way of explanation (8), the adsorbent is assumed to contain capillaries open to the surface. At low pressures, the neck of the capillary will fill, but the main body will not fill until high relative pressures are reached. On desorption, the pore will not empty until the pressure is reduced so far that the liquid in the neck of the capillary becomes unstable. According to this theory, true equilibrium corresponds to adsorption since the adsorbate contained in the main body of the pore is in equilibrium with the vapor. Hysteresis will or will not occur according to the average radius of the capillaries. Under this classification, adsorbents fall into four groups: (1) Pores are too small for molecules to enter, therefore no
hysteresis, e.g., bone charcod. (2) Small pores (10 A). Hysteresis is exhibited for smaU molecules, such tts water, but not for larger ones, such a8 benzene. Some silica gels are of this type. (3) Larger pores. Hysteresis occurs for all molecules. Other silica gels fall into this group. (4) Very large pores. No hysteresis is shown for any molecules, because the pores can be considered simply as indentations in the surface.
Silica gel falls into both groups 2 and 3. Surfaee Area. The specific surface area, A, of the silica gel can be calculated by:
where No is Avogadro's number, W is the weight of the silica gel sample, and s is the area covered by one benzene molecule. If the benzene is assumed to be flat on the surface and arrang$d in a square pattern with a lattice constant of 6 A (estimated from bond lengths), the area covered by one benzene molecule is 36 A2. If hexagonal close packmg of benzene were assumed, a smaller
Treatment of Data
Six forms of isotherms are generally observed, as shown, for example, in (8). I n 1919, Langmuir developed a theory of adsorption which described the simplest of these curves. His assumption of monolayer adsorption is of particular importance. It is often valid. However, many isotherms are for multilayer adsorption. One of the first successful attempts to give a mathematical description of multilayer adsorption was given by Brunauer, Emmett, and Teller in 1938, the BET equation (9). A later treatment known as the BDDT (10) equation, takes into account capillary and surface interaction and describes almost all of the observed isotherms.
00 1 0
2
4
6
8
Pressure icm. Hg.) Figure 2. Observed isotherm, millimoler of benzene versus prerrure in cm Hg at 23.S9C: open circler, ascending points; closed circles, descending pointr L is the number of millimoler per unimolecular layer from the Langmuir treotment; B, from the point of inflection; and BET from BET treatment.
Volume 42, Number 2, February 1965. / 89
value would be calculated. I n a review of earlier work, Livingston (11) averages [s(benzene)/s(nitrogen)] values of 1.68 and 2.13 measured on T i 0 to obtain s (benzene) = 32.3 As. This can be compared to the value 30.6 A 2 obtained from the lattice parameters of solid benzene (12) and other methods (13). The number of moles in a monolayer, n,, can be obtained first from the Langmuir Isotherm: P l n = l/n,b
+ Pin,
(2)
where P is the pressure and b is a constant. A plot of P / n versus P gives a straight lime with a slope I/%. This is shown in Figure 3. From the BET equation [Ref. (I,$), p. 1471 Pln (P
- PO)= 11-k
+ Kk - l)l%kl PIPo
(3)
where k is a constant and Pois the equilibrium vapor pressure. A plot of the left side versus PIPoshould give a slope of (k-l)/n,k and an intercept of l/%k. This is shown in Figure 4. Finally, the inflection point, "B", of the horizontal part of Figure 2 may be used as an indication of n, (14).
with the plateau ["point B" (12)l of Figure 2 seems to be high, probably because of the non-applicability of Langmuir's assumptions. Figure 3 shows a linear segment only between P/Poof 0.05 and 0.35. This may be expected [Ref. (8), p. 1581. The silica gel surface may be assumed to have a few very active sites. At low pressures these are filled first. When they are filled, a straight line plot is obtained. The deviation a t high pressures is due to the break down of the implicit assumption that there will be an infinite buildup of layers. This is taken into account in the BDDT equation which should, therefore, give a better fit.. ....
Discarding the value from the Langmuir treatment which does not seem to be applicable, the surface area obtained agrees well with that obtained by the manufacturer. These surface areas themselves probably were obtained using the BET method for adsorption of nitrogen. This indicates, barring any changes in the silica sample due to surface contamination or to accidental selection of either fine or coarse particles, that the choice of a surface area for benzene is comparable with that for nitrogen. Presumably chemical treatment of the surface, such as heating to about 400°C to destroy the -OH groups would change the adsorption. Acknowledgments
Professor R. L. Burwell, Jr., designed the prototype version of this experiment and gave us the benefit of many comments and discussions. This experiment was prepared by Mr. Hauson as an undergraduate independent study project with the aid of R. A. Sandsmark, teaching assistant. Valuable suggestions and contributions were made by students in the physical chemistry course, especially J. P. Bays, W. B. Hammond, Jr., P. B. Tjok, C. L. Fine, G. A. Mellinger, and L. 0. Crosby, 111. Part of the equipment was purchased with funds from a NSF undergraduate teaching equipment grant. Table 1.
Figure 3. BET plot, equation (3). Am expanded plot of the linear portlon, indicoted b y the brocket, was used to determine the number of moles per ~nimolecdmrlayer.
The number of milli-moles per monolayer and the specific surface areas obtained by each of the methods cited are given in Table 1. Discussion
The isotherm obtained, Figure 2, seems to be that called type I1 (8). No hysteresis !i observed, which is reasonable when the pore size, 32 A, is compared with the length of the molecule, 6 A. Other types of isotherms shown in the literature (Zb, 8) depend on the nature and the previous treatment of the surface as well as the pore size. Although the isotherm is not of the Langmuir type, a plot of equation (2) gives a straight line. However, the value of n,,, derived from the slope, when compared 90 / Journal of Chemical Education
S~ecificArea of the Silica Gel
Method
Mmolesl monolayer
Soecific areaa [m2/gml
"B" point (Fig. 2) Lmgmuir (Fig. 3) BET (Fig. 4) Manufacturer's Spec.
1.05 1.22 0.82
810 941 633 600-700
Assuming benaene area =
... 36 b2.
Literature Cited (1) (a) KISELEV, A. V., AND FROLOV, B. A., Kinetika i Kataliz (Kinetics and Catalysis) 3, 767, 774 (1962); KUNAUA. P., ibid., p. 583ff. ( b ) LAMBERT, C. F.,AND KUKHOV, CLARK, Proc. Roy. Sac. (Lon.) IZZA, 497 (1929) (appara- tus and isotherms). (2) (a) ANDERSON, J. S., Z. PhySik. C h a . , 88, 212 (1914). Includes benzene silica eel isotherm with hvsteresis. ( b ) BARTELL, F. E., AND ~ ~ W EJ.RE., , J. C O ZSei., ~ 7, 80 (1952), "Adsorption of Vapors by Silica Gel of Different Structures," benaene isotherm at 26.5'. (c) Inst. Gwkhim, Zh. Fiz. Kim, 29, 633 (1955), in Russian. Benzene isotherm at 20". ( d ) EVERETT,D. H., AND STONE,F. S., editom, "The Structure and Properties of Porous Materials," Butterworth's Scientific Pub., London, 1958. Good article on theory of benzene silica gel D. M. AND adsorption by A. V. Kiselev. (e) H~RVAT,
SING,K. W., J. Appl. Chem. (London), 11, 315 (1961). "The surface properties of silica gels: I11 adsorption of benzene and ethanol vanors." (f) RON. A,. FOLMAN. O., hem. phg8., 36, 2 4 9 , (1962): M., AND SCHNEPP, Spectrophotometric study of benzene adsorption on quartz. (3) S E O E M . ~D.~ ,P., AND GARLAND, C. W., 'rExpedmentS in Physical Chemistry" McGraw-Hill, New York, 1962. (4) JOY, A. S., Vacuum, 3,254(1953). (5) D I W I E ~F.. , ET AL., "Experimental Physical Chemistry," 6th ed., New York, McGraw-Hill Book Co., Inc., New York, 1962. (6) DUSEMAN,SAUL, "Scientific Foundations of Vacuum Technique," 2nd ed., (editor, LAFFERTY,J. M.) John Wiley and Sons, New York, 1962.
(7) "Selected Values of Physical and Thermodynamio Prop-
erties. . ." Carnegie Press, Pittsburgh, 1953, p. 362. (8) B R U N A ~ R S.,, "The Adsorption of Gases and Vapors" Vol. 1, Princeton University Press, Princeton, N. J., 1943. (9) BRUNAUER,S., EMMEIT,P., AND TELLER,E., J . Am. Chem. Soe., 60, 310 (1938). (10) BRUNAUER, S., ET AL., J. Am. Chem. Sac., 62, 1723 (1940). H. K., J . Call. Sn'., 4,447 (1949). (11) LIVINOSTON, S., J. Am. Chem. Soc., 59, (12) EMMETT,P., AND BRUNAUER, 1553 (1937). ROY.Sac., (13) PALMER,W. G., AND CLARE, R. E. D., PTOC. 149A. 360 1935. Area of benzene on elass oowder.
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