555
HEATSOF ADSORPTION ON CARBON BLACKS AND BONEMINERAL
complex along with rapid reorientation processes. The present results, although not confirming Cossee’s picture of the active centers,” are much more readily adaptable to his theory since the latter does not invoke
chemical compound formation between A1 and Ti(II1). Acknowledgment. The authors wish to express their thanks and appreciation to Dr. Charles P. Poole, Jr., for his help and advice.
Determination of Heats of Adsorption on Carbon Blacks and Bone Mineral by Chromatography Using the Eluted Pulse Technique
by R. L. Gale and R. A. Beebe Department of Chemistry, Amherst CoEZege, Amherst, Massachusetts
(Received September SO, 1963)
Isosteric heats of adsorption have been derived from the temperature dependence of retention times for eluted pulses measured in gas-solid chromatography, enabling us to make a comparison with heats of adsorption determined by calorimetric measurements a t low coverage. Chromatographic heat data were obtained for various systems using nitrogen, oxygen, argon, methyl and ethyl chlorides, ammonia, water, carbon dioxide, methane, sulfur hexafluoride, hexafluoroethane, and octafluoropropane as adsorbates and, as adsorbents, bovine bone mineral (“Ossar”) and a series of carbon blacks, Spheron, Graphon, and the graphitized forms of Sterling FT (Sterling FT-G) and Sterling S4T (Sterling RST-G). With a few understandable exceptions, the agreement of our chromatographic data with those obtained by the calorimetric method is generally good. Our treatment of the chromatographic data assumes a linear adsorption isotherm, a condition which we have shown was usually approached, though not always strictly attained, a t the relatively low coverages studied in the present work. In ideal chromatography (adsorption-desorption equilibrium) an asymmetric peak and variation of the retention volume with pulse size are indicative of a departure from linearity of the isotherm. We have derived equations which relate the surface coverage at the end of the column and any point along its length to the known column and elution curve parameters.
Introduction In addition to the interest in its established use as a method of separation and analysis, chromatography has claimed increasing attention more recently as a promising technique for the determination of thermodynamic data. Gas-solid chromatography has already been used to measure heats of adsorption,’--6 adsorption isol;herrn~,~J and surface areas.9 Wherever comparison has been made between data obtained by the chromatographic method and the previously de-
veloped procedures, agreement has been encouragingly good. Furthermore, the accuracy, simplicity, and ~~
~~~
-
(1) S.A. Greene and H . Pust, J . Phys. Chem., 62, 55 (1958). (2) H. W. Habgood and J. F. Hanlan, Can. J. Chem., 37,843 (1959). (3) P. E. Eberly, Jr., J . Phys. Chem., 65, 68 (1961). (4) R. A. Beebe and P. H. Emmett, ibid., 65, 184 (1961). (5) A. V. Kiselev, E. A. Paskanova, R. S.Petrova, and K. D. Shcherbakova, forthcoming publication. (6) S.Ross, J. K. Saelens, and J. P. Olivier, J . Phys. Chem., 66, 696 (1962).
Volume 68, Number 3
March, 1964
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R. L. GALEAND R. A. BEEBE
speed of chromatography r--,y in certain cases, offer considerable advantages over the other techniques. A considerable body of calorimetric data has been accumulated in this laboratory during the past 15 years concerning the adsorption of gases on carbon surfaces and, more recently, hydroxyapatite surfaces. With these data directly available for reference, it was considered appropriate to attempt to redetermine the heat values in the chromatograph. It was felt that such a comparative study would provide a test of the reliability and scope of the new method. Habgood and Hanlan2 proposed the following expression relating a , the slope of the initial linear portion of the isotherm, and V,, the retention volume of the adsorbed pulse a t the column temperature T ,
where Vd is the retention volume of a nonadsorbed pulse serving to correct V , for the dead space of the apparatus and W is the total weight of the adsorbent present in the column. Because V, and Vd in eq. 1 refer to the column temperature, the temperature factor T,/Tf, omitted by Habgood and Hanlan, is necessary to correct the measurements made a t different column temperatures, to the temperature of the flow meter. Hence
li, I’d
= t , T,/ ~ T = tdFT,/Tf
~ )
(2)
where t r is the retention t,ime of the adsorbate, td is the retention time of a nonadsorbed gas, and F is the flow rate of the carrier gas at Tf, the temperature of the flow meter. Equation 1 now becomes = [(tr - t d ) F
X 273Il[TfWl
(3)
Since a! = v / p , the Clausius-Clapeyron equation may be expressed as
(y) V
=
aHi/RTO2
where A H , is the isosteric heat of adsorption. tegrating eq. 4 me obtain log a
=
+A
AHi/2.303RTG
(4) In(5)
where A is a constant. Hence by plotting log a against l/To it is possible to obtain the isosteric heat of adsorption. Throughout most of the work presented here, F was held constant a t each column temperature and the effect of the variation in Tf due to changes in room temperature was negligible. Generally, therefore, it was only necessary to plot log (tr - t d ) against l/To. The Journal of Physical Chemistry
To avoid confusion, the isosteric heat of adsorption derived by the pulse technique will henceforth be referred to as the “chromatographic heat” and the term “isosteric heat” will be reserved solely for values derived from isotherms. By way of further clarification, the terms “peak” and “elution curve” are used interchangeably throughout this paper. The foregoing treatment is strictly applicable only to linear, ideal gas-solid chromatography. Linearity of the isotherm results in symmetrical peaks and departure from this condition is indicated by asymmetry. In those cases where a Type I1 isotherm (B.D.D.T. classification)10is obeyed, peaks with sharp fronts and diffuse tails are obtained. Another criterion which may be applied to test the linearity of the isotherm is the relation of the retention volume to pulse size. If the isotherm is linear in the range of pulse sizes studied, then there should be no variation in the retention volume with change of pulse size, assuming adsorption-desorption equilibrium on the column. This follows from the fact that a t successive points on a linear isotherm the equilibrium “constant)’ of adsorption (K,) is a true constant. On the other hand, if K , decreases as the amount of gas adsorbed increases (Le., the isotherm is concave toward the pressure axis), then the retention volume will decyease as the pulse size increases. Conversely, if K , increases as the amount of gas adsorbed increases ( L e . , the isotherm is convex toward the pressure axis), then the retention volume will increase with increasing pulse size. A chromatographic technique“ recently adopted in this laboratory, similar to frontal analysis whereby a constant adsorbate partial pressure is maintained in the carrier gas stream, has made it possible to measure adsorption isotherms a t low coverages (e < 0.01, where 6 = fraction of the surface covered). The evidence obtained from these measurements has lent support to the assumption that, in the pulse method, we have usually been dealing with the linear or nearly linear portion of the isotherm. Furthermore, the chromatographic heats obtained by the pulse technique have in most cases shown excellent agreement with the isosteric values determined from the chromatographically derived isotherms a t low, coverage.
(7) 8. J, Gregg and R. Stock, “Gas Chromatography, 1958,” D. G. Desty, Ed., Butterworths, London, p. 90. ( 8 ) E. Cremer, Monatsh. Chem., 92, 112 (1961); E. Cremer and H. Huber, Angew. Chem., 73, 461 (1961). (9) F. M. Nelsen and F. T. Eggertsen, Anal. Chem., 30, 1387 (1958). (IO) S. Brunauer, L. S. Deming, W. E. Deming, and E. Teller, J . Am. Chem. SOC.,62, 1723 (1940). (11) Forthcoming publication.
HEATSOF ADSORPTION ON CARBON BLACKS AND BONE MINERAL
Experimental Carbon Black Adsorbents. The four commercial carbon blacks used in the present work are designated here as Spheron, Graphon, Sterling FT-G, and Sterling MT-GI. They were kindly provided by the Cabot Corporation Laboratories in pelletized form suitable for chromatography. They represent a series of increasing surface homogeneity in the order listed. Spheron and Graphon are described in detail in an earlier publication.12 The Spheron black used in the present work is identical with that previously diescribed. The modal diameter of the particles of this adsorbent is 285 A. with a specific surface area of 110 m.Z/g. (Values of surface areas quoted throughout this work were derived from B.E.T. measurements for the cross-sectional area of the E2 using 16.2 molecule.) This material has a “vola tile contenl,” of 5yoby weight which can be removed only by heating to temperatures well above the maximum outgassing temperature of 300-320’ used in the present work for all the carbon adsorbents. The “volatile content” consists of chemisorbed oxygen and hydrogen which doubtless are present in part as oxygen-containing organic fragments which may be expected to lend a certain polarie,ing quality to the surface. The Graphon was produced by heat treatment of the Spheron a t temperatures in the vicinity of 3000’. This treatment resulted in (1) the removal of the oxygen-containing groups on the surface, and (2) a prlocess of graphitizstion bringing a higher degree of order to the parallel layer groups of the parent substance Spheron. Thus Graphon would seem to represent a geometrica,lly homogeneous and esseiitially nonpolarizing surface. It should be added that the graphitization process caused little change in the modal particle diameter but that the specific surface area wa,s reduced from 110 m.?//g. for Spheron to 89.4 m.2/g. for Graphon. The Sterling FT-G and Sterling SIT-G blacks, which are essentially the same as those described in previous publication^,^^^^^ are adsorbents of larger modal diameter and correspondingly lower specific surface area; these are, respectively, 2250 A. and 10.0 m.2/g. for the former and 3300 A. and 7.1 m.2/g. for the latter. ‘These adsorbents were prepared by graphitization of Sterling FT and Sterling M T thermal blacks by heating a,t electric furnace temperatures. Because the exact temperature of the graphitization treatment varied somewhat with different preparations in a manner which, though unknown, was not critical, especially in the commercially prepared blacks, we have chosen to designate the Sterling blacks as FT-C and MT-G rather than inserting a specific number as,
557
-
for example, Sterling MT-3100”. Sterling MT-G ranks among the adsorbents with the most homogeneous surfaces yet studied.16 I n connection with their use in the present gassolid chromatography,‘.it should be emphasized that all four of these carbon blacks appear to be essentially nonporous. This apparent absence of fine pores minimizes the probability of any “molecular sieve” effects in the functioning of the carbon black columns. Bone Mineral Adsorbent. The bovine bone mineral, designated also as “Ossar,” was obtained through the courtesy of the hrmour Research Laboratories.16 It had been prepared by extracting the collagen and other organic matter from beef bone by refluxing in ethylenediamine. This adsorbent has been described in previous publications from this laboratory. 17v1* The actual column used had previously been employed in some preliminary experiments by Beebe and Emmett.4 Vnlike the carbon black adsorbents, the bone mineral had some fine porous structure.18 The adsorptive properties of the bone mineral have received considerable attention and are discussed in the recent publications cited above. Two notable features of its behavior are its rather specific affinity for polarizable adsorbates, such as Nz, and the readiness with which the surface oxygen-containing anions appear to participate in the formation of hydrogen bonds. The latter concept was invoked to account for the abnormally high heats of adsorption of water and methanol on the outgassed surface.@ As indicated in Table 11, measurements were made on the 500’ outgassed bone mineral, designated as Ossar (500’), as well as on water-covered and methanolcovered surfaces. The Ossar (500°)4 and the modified surfaces1* were prepared in essentially the same way as previously described. Adsorbates. The gases used in this work were all of prepurified quality, supplied by the Matheson Company. The water vapor samples were taken from distilled water degassed thoroughly in vacuo. The samples of methanol vapor were taken from methanol (12) R. A. Beebe, J. Biscoe, W. R. Smith, and C. B . Wendell, J . A m . Chem. Sac., 69, 95 (1947). (13) M. H. Polley, W. D. Schaeffer, and W. R. Smith, J . Phya. Chem., 57, 567 (1953). (14) W. D. Schaeffer, W. 11. Smith, and M . H. Polley, I n d . E n g . Chem., 45, 1721 (1953). (15) C. I’ierce and B. Ewing, J . Am. Chem. Soc., 84, 4070 (1962). (10) This material is currently available from Liiiike Oy, Turku, Finland, where it is being produced under license from the Armour Phnrmnceutical Co. (17) M. E. Dry and It. A. Beebe, J . Phys. Ch.em., 64, 1300 (1960). (18) J. M. Holmes and R . A . Beehe, Advances in Chemistry Series, No. 33, American Chemical Society, Washington, D. C., 1961, p. 291.
Volume 68, Number 9 March. f964
558
degassed by bulb-to-bulb distillation in uacuo until the vapor pressure became constant (30.7 mm. a t 0'). Apparatus. The chromatograph was constructed in the laboratory. The columns were made from 4 or 5 mm. i.d. glass tubing, between 1.5 and 2.5 m. in length, and wound into compact helices. The carbon blacks. which are very fine powders as normally prepared, had been pelletized by rolling in a ball mill without the addition of water or a binder. The 20-40 mesh size pellets were screened off and used for packing in the columns. All four pelletized blacks, Spheron and Graphon in particular, showed remarkable resistance to abrasion. We are grateful to Dr. W. R. Smith of the Cabot Corporation Laboratories for suggesting the ball-mill method of obtaining the carbon blacks in a suitable pelletized form. Furthermore, it would seem that this method may be widely applied to the production of pelletized samples from any fine powders, hence obviating the use of column supports in gassolid ~ h r o m a t o g r a p h y . 6 * ' The ~ ~ ~ ~Spheron, Graphon, Sterling FT-G and Sterling RIT-G, and Ossar columns contained, respectively, 11.3, 10.1, 23.6, 33.0, and 16.5 g. of adsorbent. Two dosers were employed throughout this work to 'introduce gas samples into the carrier gas stream. The one most frequently used was a single four-way stopcock which delivered a gas pulse of 0.10 cc. (calibrated bore volume) when turned through 180". The other consisted of two four-way stopcocks enclosing a calibrated capillary volume of 2.3 cc. The detector was a Gow-3lac katharometerZ1 contained in a dewar flask. It was used in conjunction with a Leeds and Northrup Speedomax recorder which a t maximum sensitivity produced a full-scale deflection per millivolt input. Flow control was maintained by means of a needle valve. Variations in the flow rate were normally kept within i l % . Convenient flow rates (30-40 cc./min. j , measured by a soap-bubble flow meter, were obtained with a pressure drop across the column usually no greater than 10 cm. Presumably dry helium, supplied by the Air Reduction Co., was used as the carrier gas (stated purity 99.99%, or, to put it another way, possibly 10 mm. pressure of water vapor in a full tank of helium (2000 p.s.i.)). Before entry into the chromatograph, the carrier gas was further dried by passing it through a purifying train composed of several glass traps cooled by liquid air and packed with glass wool, copper foil, or cocoanut charcoal, which had been previously outgassed a t high temperat~re.~~~~~ The range of column temperatures over which the retention time measurements were made was -131 The Journal of Phwical Chemistry
R. L. GALEAND R. A. BEEBE
to 180'. To produce column temperatures less than 0", baths were prepared by pouring liquid nitrogen directly into the dewar flask containing an organic liquid with the appropriate freezing point. By maintaining a well stirred, solid-liquid slush, a steady temperature was achieved with only small variations, a t worst less than 0.5'; along the length of the column. Uniformity of temperature along the column was normally assisted by a copper tube which fitted snugly around the column. Bath temperatures were checked by a platinum resistance thermometer. A list of bath liquids and temperatures is given in Table I.2 4 A closefitting furnace with three independently controlled heating coils was used for experiments above room temperature. I n this system, temperature variations in the central region where the column was located were less than 1O. Water baths were used for measurements a t room temperature. Table I : Cold Bath Liquids and Temperatures Temp. of bath, OC. (Pt res. thermometer)
Melting point, O C .
Cold bath liquid
Diethyl succinate Bromobenzene Diethyl oxalate Diethyl malonate Benzyl acetate Ethyl acetate Acetone n-Pentane
-20.98 f O . 0 5 -31.16 1 0 . 0 2 -40.48f0.01 -50.13 1 0 . 0 5 -51.5OfO.02 -84.05f0.07 -95.16i0.15 -131.3 f 0 . 3
-21.3 -30.6 -40.6 -49,8 -51.5 -83.6 -94.8 -129.7
from tables"
See ref. 24.
The standard experimental procedure was adopted for the measurement of retention times. (See ref. 4,for example.) (19) C. G. Pope, Anal. Chenz., 3 5 , 654 (1963). (20) E. Cremer, Angew. Chem., 71, 512 (1959). (21) Model 9193 ( T E - l l ) , Gow-Mac Instrument Co., Madison,
N. J. (22) At times, a white crystalline solid which later melted t o a colorless liquid was observed t o form in the capillary tubing a t the entrance to the column when the low temperature baths were used. As the condensate was never observed a t the column exit, its source could not be traced to t h e flow meter. Qualitative tests indicated the presence of water in the condensate and presumably the condensate was essentially all water. M7e found t h a t , in spite of these difficulties with drying the carrier gas, the retention times were always closely reproducible. Doubtless, the high total surface areas of the adsorbents minimized t h e effect of the impurity. (23) R . Berry, "Gas Chromatography, 1962," paper 24, M. van Swaay, E d . , Butterworths, London. Berry discusses the stringent precautions necessary to maintain a 99.999% pure helium carrier stream. (24) Melting points were taken from the "Handbook of Chemistry and Physics," 42nd Ed., Chemical Rubber Publishing Co., Clevel a n d , Ohio, 1960-1981.
559
HEATSOF ADBORPTION ON CARBON BLACKS AND BONEMINERAL
-
Table I1 : Heats of Adsorption of K2, 0 2 , Ar, and CHI on Bone Mineral, Spheron, Graphon, Sterling FT-G, and Sterling MT-G
Adsorbate
Nz
---Bone Outgassed at 500°
-----
Chromatographic heats of adsorption, kcal./mole-------Carbon blacks-----mineral (Ossar)---MethanolWaterSterling covered covered Spheron Grhphon FT-G
--
Sterling MT-G
5.3 2.1b 4.0
2.7 2.6 2.4 0 2
3.0 2.0 4.0
2.7 2.6 2.5
Ar
2.5d 2.lb 3.9 2.7 2.6 2.5
CH4
3.4 3 .0
Temperature range of retention time measurements, OC.
- 84 to 0 -131 tQ -95 - 31 t o 28 -131 t o -51 5 t o -95 -131 -131 t o -84 - 84 t o 0 -131 to -95 - 31 t o 28 -131 to -51 5 -131 t o -95 -131 to -84 - 84 t o 0 -131 to -95 - 31 t o 28 -131 to -51 5 -131 t o -95 t o -84 -131 - 51 5 to 0 -131 to -78
Calorimetric heats of adsorption, kcal./mole a t -195' 8 = 0
5 . 5 f 0.3° 3.0a 4.40 2.gC
4
46
3.5"'d 2.4" 3 . 8'~* 2 , 6".'
It is possible that we obtained a more homogenous surface than did Holmes and Beebe,'* which would account for a See ref. 18. the lower chromatographic values. c See ref. 12. d The lack of agreement between chromatographic and calorimetric values is disappointing in this case. Unlike nitrogen, the calorimetric work with argon on Ossar (500") was limited t o one series of differential heat measurements. It IS possible that the data for the first two increments of that series may be in error. (See Holmes and Beebe.'*) e See ref. 25. See ref. 26.
-
-
Results and Comparison with Earlier Calorimetric and Isosteric IData Data for Nitrogen, Oxygen, and Argon. Retention time studies were made for pulses of nitrogen, oxygen, and argon, on each of the four carbon blacks and on the bone mineral (both outgassed and methanolcovered surfaces). The pulse sizes were less than 0.15 cc. (STP), except for Sterling FT-G where the larger doser was used to deliver pulses of 2.16 cc. (STP). Typical plots of log (t, - td) us. Il/T, are shown in Fig. 1, 2, and 3 for Ossar (500°), Spheron, and Graphon, respectively. It is seen that the experimental data lie reasonably well on straight lines. With the possible exception of nitrogen on Ossar (500°), it seems probable that any deviation from a straight line relationship may be due to minor variations in the temperature of the ambient baths in which the columns were immersed. Similar straight line plots, not shown, were obtained for each of the three elementary gases on Sterling FT-Gr and on Sterling AIT-G carbon blacks as well as on thLe methanol-covered bone mineral.
Table I1 presents a list of the heats of adsorption derived from the retention times, together with the temperature range of the retention time measurements. Calorimetrically measured heats of adsorption extrapolated to zero coverage, together with the references for the sources of the calorimetric data are given in the final column of Table II.26,26 With the exception of the data for the system nitrogen-Ossar (500°), all the values for the chromatographic heats of adsorption listed in Table I1 appeared to be independent of the pulse sizes used. However, the pulse size did have an effect on the intercept of the log (t, - t d ) us. 1 / T , plot in some instances. Data for Methyl and Ethyl Chloyides. A retention time study of methyl and ethyl chloride pulses was made on both Graphon and Sterling >IT-G. Three pulse sizes were used for each gas. The plots of log (t, - td) us. l/Tc for the alkyl chlorides on Graphon (25) R. A. Beehe, B. Millard, and J. Cynarski, J . A m . Chem. Soc., 75, 839 (1953). (26) R. A. Beebe and D. M. Young, J . Phys. Chem., 58, 93 (1984).
Volume 68, Number 3
March, 1964
560
R. L. GALEAND R. A. BEEBE
2.9
-
-
2.5 -
3 2.1 I
‘ c
-1
-
1.7-I
1.3
-
o,
-
09 -
4.0
I
I
.
l
l
l
l
l
,
*
l
,
,
5.0 103
x
T / T ~6 .O
7.0
Figure 3. Nt, 02, and Ar on outgassed Graphon: 0 , NZ (0.148 cc. (STP)); 0, OZ(0.103 cc. (STP)); 0, Ar (0.059 cc. (STP)); pulse sizes are given in parentheses. 3.4
I
2.81
/
-
3.0-
-
2.6-
-
2.2
-0
I
&
-
1.8-
0
3 3.0
3.5 103
4.0
x
-
-
4.5
1 /T,
Figure 2. Nz, 02,and Ar on outgassed Spheron: 0 , NZ(0.103 O2 (0.103 cc. (STP)); 0 , Ar (0.059 cc. cc. (STP)); 0, (STP)); pulse sizes are given in parentheses.
are shown in Fig. 4. Over the range of temperature studied, 0 to B O 0 , the adsorbates were found to be, in part, chemisorbed on both adsorbents. Chemisorption was inferred from the fact that successive gas pulses of the same size were observed to give peaks of increasing height. That is to say, a decreasing fraction of each successive pulse remained on the column. A constant peak height was attained after no more than two or three sample injections, however. As further evidence of chemisorption, ‘‘foreign” displacement peaks werc obtained in subsequent runs with different adsorbates when the chemisorbed fraction was displaced from the column. Those retention times obtained a t constant peak height were used in the derivation of heat values. Nooi, Pierce, and Smith2’ measured the isosteric heats for ethyl chloride on Graphon The Journal of Physical Chemistry
1.4
2.0
2.5
3.0 103 X l / T o
3.5
Figure 4. CH3Cl and CzH6C1on outgassed Graphon. C&Cl: 9 , 0.003 cc. (STP); 8 , 0.020 cc. (STP); 0 , 0.119 cc. (STP). CHsCl: 0 , 0.003 CC. (STP); 0 , 0.014 CC.(STP); 0 , 0.197 cc. (STP); pulse sizes are given in parentheses.
over the temperature range -78 to 75’. The value obtained from their results by extrapolation to zero coverage was 8.5 kcal./mole. Tables 111 and I V summarize the comparative heat data for the alkyl chlorides on Graphon and Sterling >IT-G. It is noteworthy that the heat values for the two chlorides are consistently lower on Sterling MT-G than on Graphon, so that again we have evidence for the greater homogeneity of the former material. Data for Ammonia and Water. A retention time study was made of pulses of ammonia on Graphon and Sterling hIT-G, and water vapor on Sterling NIT-G. (27) J. Mooi, C. Pierce, and R. N. Smith, J . PIkys. Chem., 57, 657 (1963).
561
HEATSOF ADSORPTION ON CARBON PT~ACKS AND BONEMINERAL
Table 111: Heats of Adsorption of CHaC1, C2H6C1,NH,, and (202 on Graphon
Adaorbate
CH3C1
CzHsCl
8"
coz
Chromatographic heats of adsorption, keal. /mole
6.5 6 4 5.7 8.9 8.3 7.7 Between 4 . 5 and 3 . 9 Between 4 . 2 and 4 . 0 4.4 4.4 4.3
Temperature range of retention time measurements,
oc.
19.5 to 179 0 to 179 0 to 179 102 to 179 72 to 179 72 to 179 23 to 100 23 to 100 0 to 5c 0 to 50 0 to 50
Isosteric and calorimetric heats of adsorption, koal./mole a t B = O
Pulse &e, ec. (STP)
0.003 0.014 0.197 0.003 0.020 0.119
t
8.5" ( - 78 to 75")' 7 . 2 (-79"). 7.3-4.5 (0")C
0.197 0.003 0.103 0.197
4 . 4 (-79o)dme 6 . 3 (-79O)"l'
a If the heat of condensation of CzH6Clai, 120" (-5.7 kcal./rn Dbtained by extrapolationz7)is added to the net heat of adsorption measured by Mooi, Pierce, and Smith over the temperature range -78 to 75", the value 8.2 kcal./mole is obtained for the total heat of adsorption. See ref. 27. 0 See ref. 28. d COS on Sterling MT-G. e See ref. 29. COZon Sterling FT-G.
Table IV: Heats of Adsorption of CHBCI, CZH5C1, NHa, and H20 on Sterling MT-G
Adsorbate
CHIC1
CzHsC1 a"
Ha0 SFs
CZFO C3Fs
Chromatographic heats of adsorption, kcal./mole
5.7 4.7 4.5 6.6 6.3 5.2 4.4 4.0 Between 6.0 and 5 . 3 Between 5 . 3 and 4 , 5 5.2 4.9 7.2
Temperature range of retention time measurements, OC.
75 to 140 75 to 140 75 to 140 75 to 140 75 to 140 75 to 101 75 to 121 75 t o 121 32 to 96 32 to 96 -83 t o -37 -83 to -37 -36 to -24
Pulse size, cc. (STP)
Calorimetric heat of adsorption, kcal./mole a t
e-o
0,002 0.103 0.191 0.103 0,019 0.119 0.187 0,040 0 090 0.090 0.090 I
7 . 5 (-78 to 0 ° ) 4
t
7.1 (-79O)O Between 10 and 6d 4.9 i 0 . 3 (-78")"
5 I f the heat of condensation of CzH&l a t 100" (5.7 kcal./mole, obtained by extrapo lationZ7)is added to the net heat of adsorption measured by Pierce and Ewing over the temperature range -78 to 0") the value 6.5 kcal ./mole is obtained for the total heat of adsorption a t 100'. See ref. 15. See ref. 29. See ref. 38. e See ref. 32.
A summary of comparative heat data for these systems is contained in Tables I11 and IV. The chemisorptive phenomenon mentioned above in connection with methyl and ethyl chlorides was also observed in the case of ammonia, though it was not detected with water. As with the alkyl chlorides, the first two or three pulses were sufficient to saturate the surface with the irreversible fraction; thereafter, the peaks became constant in size, and the retention times which were then measured were subsequently used to derive the chromatographic heat.
The points on the plots of log (tr - t d ) us. l/Tc for ammonia on Graphon showed some scatter, particularly a t the higher temperatures (where t d 3 tr). It was therefore impossible to abstract with confidence a single value of the chromatographic heat from the graphs for each pulse size used. We have preferred to express the results as a range of values somewhere within which we believe the true value lies. In the case of ammonia on Sterling MT-G, experimental points were obtained a t only two temperatures for the 0.019 and 0.119 cc. pulses and at three temperatures Volume 68,Number 9 March, 1964.
562
R. L. GALEAND R. A. BEEBE
for the third pulse studied, 0.187 cc. Although the the column of Sterling SIT-G was placed in a cryostat results are shown in Table IV as single values, we do built from the design of Graham.31 In the pulse not wish this to be interpreted as an indication that studies made with these fluorine compounds, the flow we regard these figures with the same confidence as rate of the carrier gas was purposely allowed to vary we regard, for example, the values for the chroma(from 50 to 30 cc./min. a t room temperature) so that tographic heats set down in Table 11. Dell and Beebe28 we could learn whether or not the retention volume was measured the calorimetric heat of adsorption for dependent on the flow rate. The fact that we obtained ammonia on Graphon at -79". They found that the good straight lines when we plotted log a us. 1/T, heats dropped sharply from 7.2 to 6.0 kcal./mole while indicates that, within the range of values investigated still a t low coverage. The accuracy of the caloria t any rate, the choice of a particular flow rate was not metric measurements which they made a t 0' suffered critical, an assumption we had made throughout this work. The log a us. 1/T, plot for sulfur hexafluoride owing to the greatly reduced adsorption; here the initial value, measured at 0 = 0.025 was 7.2 kcal., may be seen in Fig. 5. dropping to 4.5 a t 0 = 0.08. (The latter value is less than the heat of condensation at O", namely, 5.1 k ~ a 1 . ~ ~ ) t Spencer, Amberg, and BeebeZ9measured the calorimetric heats of adsorption for ammonia on Sterling MT-G a t -79'. The results were very similar to those obtained on Graphon. It is difficult to make a quantitative comparison of the calorimetric and chromatographic heats, although the latter do seem to run lower than the former. Little difficulty was experienced in obtaining water vapor peaks over the range of temperature 32 to 96', using two pulse sizes, 0.017 and 0.04 cc. The heat data for water are set down in Table IV. The log (t, - t d ) us. 1/T, plots for the water pulses did not produce good straight lines. As in the case of ammonia on Graphon, therefore, we have acknowledged our uncertainty in the derivation of chromatographic heats of adsorption for water by expressing the results in the form of a range of values within which we think the true 4.0 4.5 5.0 value lies. The chromatographic heat of adsorption 108 X l / T o for water is seen from Table IV to lie somewhere in Figure 5. Sulfur hexafluoride on Sterling the region 6.0 to 4.5 kcal./mole, which is well below MT-G; pulse size, 0.09 cc. (STP). the heat of condensation (10.5 kcal. a t 32' and 9.7 a t 96'. Neither calorimetric nor isosteric data have been Data ,for Additional Adsorbates. Pulse studies were measured specifically for carbon dioxide on Graphon. also made of methane on both Ossar (500') and ivaterHowever, Spencer, Amberg, and BeebeZ9made measurecovered Ossar, carbon dioxide on Graphon, and sulfur ments on both Sterling FT-G and Sterling NIT-G a t hexafluoride, hexafluoroethane, and octafluoropropane -79". The chromatographic values shown in Table on Sterling SIT-G. The derived heat data for methane I11 agree better with the data for the latter, where the are shown in Table TI and the plot of log (t, - ta) us. initial heat was 4.4 kcal./mole, than for the former, l / T ofor methane on Ossar (SOOo) is included in Fig. 1. where the heat dropped sharply from an initial value Three different pulse sizes of carbon dioxide were investigated and the chromatographic heats are included (28) It. M. Dell and R. A. Beebe, J. Phys Chem., 59, 754 (1955). in Table 111. The logarithmic dependence of cor(29) W. B. Spencer, C. H. Amberg, and R. A. Beebe, ibid., 62, 719 rected retention times upon 1/T, was closely linear for (1958). the pulses of methane and carbon dioxide on their (30) We are indebted to Dr. R. F. S. Tyson for his part in the respective adsorbents. The results of the pulse method measurement of these d a t a , particularly for octafluoropropane. experiments for sulfur hexafluoride and the fluoro(31) D. Graham, J . Phys. Chem., 66, 1815 (1962). We have found t h a t under favorable conditions this cryostat is capable of temcarbons on Sterling MT-G30 are listed in Table IV. perature control within 0.02O and Graham has claimed t h a t control In the work done with these last-mentioned adsorbates, may be obtained which is ten times better than that. ~~
The Journal of Physical Chemistry
563
HEATSOF ADSORPTION ON CARBON BLACKSAND BONEMINERAL
of 6.3 kcal. to 4.8. No previous study of methane on a hydroxyapatite surface has been made. A calorimetric investigation of the large, symmetrical, nonpolar molecule, sulfur hexafluoride, on Sterling MT-Ci was made by E. R. Camplin in thii3 laboratory. 3 2 Camplin’s initial calorimetric value was measured a t a sufficiently high coverage so as to render somewhat uncertain the extrapolation to 8 = 0. Our best estimate of this extrapolated value is 4.9 f 0.3 kcal./mole.
Discussion Some Sources of Uncertainty in Comparison of Data. Wherever possible, the calorimetric data to be cited in the sections below were obtained by extrapolation to zero coverage, which is believed to be reasonably comparable with the low coverages attained by a pulse as it passes through the column. Accurate extrapolation of calorimetric data to zero coverage is a difficult procedure because each “differential” point represents, in fact, a finite area of covered surface. I n many cases the slope of the graph showing the differential heat as a function of coverage is greatest in the low coverage region, for which most likely only two or perhaps three points have been determined. Extrapolation may then easily lead to inaccuracies of a few hundred calories. The chromatographic and calorimetric data should be corrected to a common temperaiture. In order to convert Calorimetric heats of adsorption a t temperature T I to another temperature T 2 ,C, and Cad, the heat capacities of the gas in the gas phase and adsorbed phase, respectively, must be known. The value of (C, - Cad) wa,s not known for any of the systems we studied so that it was not possible to make this correction. As an example of a well studied system, however, we may refer to the work by Drain and Rlorwith nitrogen adsorption on rutile. They have shown that the difference (C, - Cad) is small (perhapa less than 1 cal./mole deg.) between 78 and 125OII. Kuelllnans, ref. 39, g . 106 ff. (41) Calculation showed t h a t t h e peak width represented a distance
actually greater than that between the doser and the katharometer. T h e front of the penk was very sharp so, presumably, the initial katharometer response was fast. However, i t would nppenr t h a t the gas could not diffuse away sufficiently rapidly froin the kathnrometer chamber for, although the rear of the penk fell back sharply a t first, there was some tailing ns it nppronc,hed the hnse line. T h e katharometer used in our work was a high flow stability Gow-Mac instrument*' whose detector wire was located i n n recession set in at right angles t o the main carrier stream. This type of knthnrometer is very coninio~iiyemployed in chromatogmrJhy.
Volume 68, Number S
M a r c h , 196L
566
R. L. GALEA N D R. A. BEEBE
Table V: Surface Coverages 2 in om. from
Pulse Temp., Adsorbate
N2 0 2
Ar
KZ 0 2
Ar
KZ 0 2
Ar ?;z 0 2
Ar
KZ 0 2
Ar SFfi CzFfi C3Fs
Adsorbent
Graphon Graphon Graphon Sterling FT-G Sterling FT-G Sterling FT-G Sterling MT-G Sterling MT-G Sterling MT-G Ossar (500O ) Ossar (500') Ossar (500O ) Spheron Spheron Spheron Sterling MT-G Sterling MT-G Sterling MT-G
OC.
- 60 - 60 - 60 - 95 - 95 - 95 - 95 - 95 - 95 - 60 - 60 - 60 0 0 0 -82 - 80 -28.7
Ir
size, cc. (STP)
0.45 0.40 0.38 0.17 0.16 0.14 0.23 0.20 0.19 3.70 0.30 0.22 0.87 1.40 0.70 28.5 24.0 13.3
0.15 0.10 0.06 2.16 2.16 2.16 0.10 0.10 0.10 0.15 0.10 0.06 0.10 0.10 0.06 0.09 0.09 0.09
Ofm,, by use of eq. 11. On the other hand, the non-
linear conditions prevented an accurate evaluation of Oz,,. Severtheless, we believe that the values of OZ, for these systems given in Table V have a t least a qualitative significance. The very low Reynolds number ( < 5 ) encountered under usual conditions indicated that the flow was laminar in the capillary section leading from the doser to the column entrance. Some pre-column pulse broadening would have occurred as a result of the parabolic flow profile assumed by the puke and as a result of molecular diffusion. We conclude, therefore, that although an unexpectedly high coverage (e > 0.1) may possibly prevail initially, it is rapidly reduced as the pulse advances along the column. There would appear to be some advantages in deliberately allowing the pulse to suffer some pre-column mixing in order to reduce a relatively high, initial coverage; alternatively, smaller pulses could be used. Somewhat larger retention times [(tr - td) .> 5 min.] than were usually measured in this work are also recommended, whercver it is possible to achieve them. Val-iation of Retention Volume with Pulse Size and the Iinearity of the Isotherm. I t has already been pointed out in the Introduction that a variation of retention volume with adsorbate pulse size is indicative of a noillinear isotherm, assuming that we have ideal chromatographic conditions. Several plots of the variation of retention volume with sample pulse size are shown in Fig. 7 . It will be noted that there is a The Journal of Physical Chemistry
elmax
0.0022 0.001 0.0008 0.05 0.028 0.030 0.0074 0.0024 0.007 0.003 0,001 0.001 0.0006
0.00022 0.0006 0.015 0.011 0.022
column entrance
elmax
2.0 2.0 2.0 1.8 1.8 1.8 2.5 2.5 2.5 2.0 2.0 2.0 2.1 2.1 2.1 2.5 2.5 2.5
0.022 0.010 0.008 0.50 0.28 0.30 0.074 0.024 0.070 0.03 0.01 0.01 0,006 0,0022 0.006 0.15 0.11 0.22
marked decrease in a few cases and, moreover, an increasing departure from linearity as smaller pulses were used. The results obtained by the frontal analysis" method have given us direct proof that we have generally been dealing with the linear, or nearly linear, portion of the isotherms. Such results have so far been obtained for the systems: argon on Ossar (500') between -64 and -86' in the region of coverage, 0 = 0.0001 to 0.003; nitrogen, argon, and oxygen on Graphon over the temperature range -75 to -96' in the region of coverage, e = 0.0001 to 0.004; and nitrogen on Sterling 3lT-G in the temperature range -84 to -102' and e = 0.0025 to 0.02. Furthermore, the isosteric heats derived from these isotherms agree closely in each case with the retention time heat data. On the other hand, the isotherms obtained for nitrogen on Ossar (500') between -50 and -76' over the coverage range e = 0.001 to 0.07 were distinctly concave toward the pressure axis, particularly a t the lowest temperature. This result was anticipated from the increasingly asymmetric peaks obtained as the column temperature was lowered and the marked variation of retention time with pulse size shown in Fig. 7 . Xevertheless, the isosteric heats a t low coverage extracted from the isotherms were not very much greater (-0.7 kcal./mole) than the retention time value stated in Table 11, and the latter value agrees very well with the calorimetric figure. On account of the dependence of retention time on
567
HEATSOF ADSORPTION ON CARBON 13LACKS AND BONE MINERAL
to study chromatography in her institute. Our gratitude is due to the National Science Foundation whose financial support enabled us to undertake this research.
Appendix Habgood and Hanlan2 gave no details of the derivation of eq. 1 in their paper. We feel it is of interest to show how this equation has been derived. The equilibrium constant for the adsorption process in gas-solid chromatography is usually defined in the following way.
Ka = [als/[alg
(16)
where
bl.
=
cc. (STP) adsorbate g. of adsorbent
and cc. (STP) adsorbate [a', = unit volume gas a t column temp. T,, containing adsorbate a t partial pressure p
--o-
The slope of the initial linear portion of the isotherm, a,is given by
CY = v / p (17) I where ZJ is the volume of gas adsorbed in cc. (STP)/g. 0.I 0.2 Pulse Size in cc. STP of adsorbent and p is the corresponding equilibrium
--
Figure 7. Variation of retention volume with sample pulse size: n, Nz on outgassed bone mineral at -78'; V, Ng on Sterling MT-G at -98.8'; V, CHsCl on Sterling MT-G a t 101"; A, HzO on Sterling MT-G a t 71'; ab) NH, on Sterling MT-G at 75.5"; 8 , Ar and 0 2 on Sterling MT-G at -94"; a, CH&l on Graphon a t 122.3"; 0 , CsHd21 on Graphon at 122.3'; 0, Ar on Graphon at -95.5'; A) NZon Graphon at -95.5".
pressure. Hence
KfL[alg/p (18) By substituting the term 273p/T,, obtained by application of the gas law for [ a ] ,in eq. 18, we obtain =
Ka273/Tc (19) By appropriate modifications to the analogous equation for gas-liquid c h r o m a t ~ g r a p h ythe ~ ~ following expression is obtained. cy
pulse size shown in Fig. 7 , we wish to endorse the note of caution sounded by Beebe and Emmett4 concerning the prospects offered by gas-solid chromatography for analysis,
Aclcnowledgments. We wish to express our indebtedness to Prof. L. Willard Richards of this department for his invaluable advice upon the 1,heoretical aspects of gas-solid chromatography (particularly in the compilation of the section on surface coverages) and to Prof. Dr. E. Cremer of the Institute of Physical Chemistry, University of Innsbruck, Austria, for kindly permitting R. L. G., recipient of a N.A.T.O. Fellowship,
Ka
=
= (vr
-
Vd)/W
(20)
The retention volumes V , and v d should be converted to limiting values for zero pressure drop across the col~ r n n . (In ~ ~ the work described in this paper the pressure drop across the column was always sufficiently small to make this correction negligible.) Substituting for Ka in eq. 19 from eq. 20 we obtain eq. 1. (42) See Keulemans, ref. 39, p. 112, eq. 12. (43) A. T. James and A. J. P. Martin. Biochem. J.,50, 679 (1962).
Volume 68, Number 3 March, 1961
