Oct., 1957
PORESIZEDISTRIBUTION OF POROUS ALUMINUM OXIDES
1417
curate to better than a few per cent. About all that can be said regarding the comparison is that our results appear to fall within the range of values previously reported. None of the results obtained have been compared with those listed by Hutchinson.2 A careTABLE I11 ful examination of his paper reveals a startling number of inconsistencies. It is stated, for exCOMPARISON OF THERMAL CONDUCTIVITIES K ,cal. cm.-* sec.-1 'C.-' X 106 ample, that the temperature differences across Ref. This reRef. 8 Ref. Ref. 10 the cell during energy input were of the order of search (0") 7 9 (15') one degree or less, while his plot of the data indiMethyl alcohol 54.9 50.5 5 2 . 4 50 47.5 cates AT'S of ten degrees or more in some cases. 42.6 43.0 44.6 41 Ethyl alcohol These larger temperature differences, moreover, Isopropyl alcohol 34.7 36.7 36.8 are compatible with the range of power inputs 34.1 2 6 . 6 37 25.6 Carbon tetrachlogiven. The method of treating the data appears ride to be unnecessarily complicated and not enough inChloroform 42.7 30 28.9 formation is given so that the work can be evalAcetone 53.4 42.9 42.3 45.3 uated. For these and other reasons, the data of It can be seen that the agreement between pre- ref. 2 are not included in the comparison. vious investigators is not very satisfactory. ReadExperimental Errors.-The principal systematic ers are referred t o the literature for a description error in the dimensional factors is believed to be of the various methods used. Perhaps the best associated with specifying the effective length of agreement exists between the results of Bridgman* the helical filament. Although this length can be and Goldschmidt.8 However, in ref. 7, Bridgman measured readily to within O.l%, it is conceivable has used his own temperature coefficient data to that this may differ from the "true" effective correct the results of Goldschmidt to 30' in order length by as much as 1%. The radius of the helix to make a comparison. I n his compilation, which should be accurate to well within 1%. However, includes the liquids listed here and others, there is an error of 1% in the radius is equivalent to an good agreement in some cases but differences of error of only 0.5% in the final results. Systematic 10% or more in other cases. Bates,g on the other errors in the other dimensional factors, in the corhand, reports temperature coefficients for these rections applied to those associated with the liquids that are greater than those of Bridgman by operation of the auxiliary equipment are believed a factor of ten or more. It is evident that much to be well under the limits imposed by the helix careful work must be done before any liquid ther- length and radius. From these considerations and mal conductivity values can be accepted as ac- from an estimate of random error, the results are believed to be reliable to within 3%. (8) R. Goldschmidt, Physik. Z.,12, 417 (1911). obtained on different samples of each liquid with the results reproducible t o within 1% or better. The comparison is given in Table 111. The results of this research and all others are for 30" unless otherwise specified.
(9) 0.K. Bates, G . Haazard and G. Palmer, Ind. Bng. Chem., 33, 375 (1941); Ind. Eng. Chem., Anal. Ed., 10, 314 (1938).
(10) E. F. M. Van Der Held and F. G. Van Drunen, Physica, X V , 865 (1949).
PORE SIZE DISTRIBUTION OF POROUS ALUMINUM OXIDES BY MERCURY POROSIMETER AND n-BUTANE SORPTION BY C. N. COCHRAN AND L. A. COSGROVE Aluminum Co. of America, Alcoa Research Laboratories, New Kensington, Pa. Receiusd June 10, 1967
A mercury porosimeter, which uses a compressed oil pumping system, has been used to measure the pore size distributions of ten porous aluminum oxides. These values agree well with the distributions found by measuring the sorption of n-butane at 0' with a quartz helix balance.
Introduction The performance of alumina in many of its applications is determined by pore structure. An extensive knowledge of this structure has been gained by employing gas sorption determinations. For the anodically formed porous oxide the structure determined from sorption measurements' independently confirmed those found by the electron microscope. Another entirely independent (1) L. A. Cosgrovc, THISJOURNAL, 60, 385 (19513). (2) F. Keller, hf. 8. Hunter and D. L. Robinson, J . Electrochem.
means for learning these structures is available in the mercury porosimeter described by Drake and Ritter.a In this method, the pressure required to force measured volumes of mercury into pores is determined. From this information the volume of each size of pore in the sample can be calculated. The results of structure determination with the porosimeter for an anodically formed aluminum oxide and for several chemically produced porous aluminum oxides are compared with n-butane sorption data in this paper. Other comparisons of this
Soc., 100, 411 (1953); M. 9. Hunter and P. Fowle, ibid., 101, 514 (1964).
(3) L. C. Drake and H. L. Rittor, Ind. Eng. Chem., Anal. Ed., 17, 782 (1945).
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C.,N. COCHRAN AND L. A. COSGROVE
Vol. 61
TABLE I COMPARISON OF POREVOLUMEAND PORESIZEDISTRIBUTION IN POROUS ALUMINUMOXIDES
---
Sample Alcoa H41 Gel Alumina Alcoa Experimental Gel Alumina Alooa Experimental Gel Alumina Alcoa Experimental Gel Alumina Alooa Experimental Gel Alumina Alooa Experimental Gel Alumina Alooa Experimental Gel Alumina Alcoa Experimental Gel Alumina Anodized Aluminum Film Alcoa, F10 Alumina
Porosimeter n-Butane Porosimeter +Butane Porosimeter n-Butane Porosimeter n-Butane Porosimeter n-Butane Porosimeter n-Butane Porosimeter n-Butane Porosimeter n-Butane Porosimeter n-Butane Porosimeter n-Butane
450002000 0.006
...
,006
...
.003
... ...
.014 ,004
...
,004
...
.004
...
.011
...
,0176
...
,016
...
Volume ml /gJ between pore radius (A.) limits 20001060: 5002001001000 500 200 100 50 50-30 0.002 0.001 0.001 0.001 0.001 0.004 ... ... ,008 .004 .006 ,021 .002 .001 .004 .002 .006 ,169 ... ... ,009 .004 ,007 ,204 ,001 ,001 .002 ,015 ,009 ,304 ... .OOO ,000 .002 .218' ,005 ,005 .013 ,010 ,003 ,007 ,023 .009 .006 .015 ,001 ,001 .OOO ,000 .383 ,086 ... ... .001 ,004 .356 .003 ,015 ,004 ,005 .360 .056 ... ... .001 ,001 .275< ... ,001 .OOO ,001 ,029 .008 ,135 ... .001 .002 .005 .255 .007 .020 ,027 ,016 .015 .017 ... .025 ,012 .011 ,021 ,0007 .0012 ,0016 .0007 .0063 .0120 ... ,0011 ,0020 .0057 .0107 .008 .013 ,026 .032 .048 .067 ... ,027 ,027 .028 ,093
... ...
...
...
...
... ...
...
Volume sorbed i? pores smaller than last listed n-butane value. 30-24 A. eter and up to 500 A. for n-butane sorption. 30-23 A. e 50-32 A. f 30-20 j 500-20 A.
type were published by Joyner, Barrett and Skold4 for sorption of nitrogen on sugar refining adsorbents and by Kamakin and Kiselev5 for sorption of methyl alcohol on alumina-silica gel.
Experimental A. Mercury Porosimeter.-As suggested by Drake,s a compressed oil pumping system was used with the mercury porosimeter rather than the much more hazardous compressed gas system. The pumping system consisted of a motor driven hydraulic pump (0-3000 p d . ) , a pressure intensifier (100,000 p.s.i. limit, 23-1 ratio), and valves capable of withstanding pressures up to 100,000 p.s.i. The bomb with an internal cavity 11/* in. in diameter and 21 in. deep, was designed for a working pressure of 70,000 p.s.i. and was closed by a Bridgman unsupported area seal. The resistance of a calibrated manganin wire sealed into the system was determined to 0.00001% with a Mueller Bridge to measure the pressure to a sensitivity of 5 lb. Corrections for atmospheric pressure and the head of mercury over the sample were made to the readings. The Pyrex dilatometer used for holding the sample and the mercury in the bomb was similar to the one described by Drake and Ritter* except that the sample chamber was closed at the bottom by a 12/30 - glass joint sealed with hard de Khotinsky cement. Using a joint to seal the sample into the dilatometer rather than the usual fused glass seal had the advantage of fixing the internal volume of the dilatometer to within f 0.005 ml. for determining the sample density. The position of the mercury in the capillary of the dilatometer was determined by the uaual method of letting the mercury short across a No. 3?70% platinum 30% iridium wire doubled through the capdlary. The change in resistance of the wire from 3 ohms a t the top of the capillary to 16 ohms a t the bottom was measured to the nearest 0.01 ohm with a Wheatstone bridge. As much sample was used as possible, but the total measurable pore volume could not exceed 0.08 ml. The correction for the compressibility of the mercury, glass and wire and for a slight amount of penetration into the de Khotinsky seal was evaluated in blank runs and found. to be reproducible and linear with pressure. This Correction amounts to about 6% of the total possible volume change in going to 60,000 lb./sq. in. I n applying this correction to an (4) L. G. Joyner, E. P. Barrett and R. Skold, J. Am. Chem. SOC.' 73,3155 (1951). (5) N. M. Kamakin and A. V. Kiselev, Dokladg Akad. Nauk S.S.S.R., 83, 589 (1952). (6) L. C. Drake, Ind. Eno. Chem., 41, 780 (1949).
Re30-15 mainder" 0.357 .161b 0.258 280b .182 .102d .158 ... ,152 .216 .152d ,237 ,016' .160 ... 040Q ... ,224 .266 .080h .127 ,138 .080d ,178
.
.
,OOlOb .0038b ,042 ,037
,0037 ,072
Total pore v01.C 0.373 ,458 ,470 .508
.493 ,372 ,273 ,442 .491 ,521 .487 ,501 ,444 .470 ,251 .327 .0216i .0233< ,252 ,284
45,000-15 A. for the mercury porosimA. 0 30-26 A. h 30-22 A. 100-60 A.
actual run, the compressibility of the sample and the glass wool used to hold the sample in place was taken to be the same as for the mercury, glass and wire. Temperature corrections for the expansion of the glass and mercury and for the change in resistance of the platinum-iridium wire were also made. Readings could not be taken as pressure was decreased because of an insulating film which formed on the platinumiridium wire where it was contacted with oil. Floating water between the oil and mercury did not prevent this film from forming. The radius of the pores just enterable by mercury of surface tension u, under pressure P and a t a contact angle 0 with the material being tested is expressed by the formula: radius = 2u cos B/P. A value of 480 dynes/cm. was employed for u a t the operating temperatures. A contact angle of 130' was employed in deference to Joyner, Barrett and S k ~ l dwho , ~ obtained the best agreement between nitrogen sorption and porosimeter data on carbon using thls value. The pressure in the bomb was increased by steps to 60,000 p.s.i. stopping long enough for the readings to come to a constant value. B. n-Butane System.-Gas sorption a t 0' was measured on pelleted samples in a McBain-Bakr type quartz helix balance system employing chemically pure n-butane gas. The elongation of the quartz helix was measured with a cathetometer, equipped with a vernier which could be read to fO.OO1 mm. Gas pressures in this system were measured with a mercury manometer to zkO.1 mm. Before measuring sorptions, the samples were evacuated to constant weight a t predetermined temperatures.
Discussion of Data The comparison of the pore structures of the 10 porous aluminum oxide specimens, as determined by the mercury porosimeter and n-butane sorption, is shown in Table J, The lowest radii shown are calculated by the Kelvin equation from the point where the desorption portion of the hysteresis loop rejoins the sorption portion. The remainders shown are those volumes of gas sorbed on the sample and in the pores to a thickness of approximately 2 molecular layers before capillary condensation can occur. It is obvious from the remaining values that there are pores smaller than those listed in these materials, but at present there
Oct., 1957
THERMODYNAMIC DATAIN DONOR-ACCEPTOR FORM
are no means available for measuring the distribution in the small pore range. Although the volumes for a given range may not agree exactly by the two methods, the ranges in which volume is concentrated check by the two methods. I n only two of the nine samples investigated by these two techniques was there any app:eciable number of pores found above the 300-50 A. radius range and these were in the 50-100 A. range. With only the anodized aluminum film did it appear that all of the available pores had been investigated. I n the remaining nine samples there were probably more pores at smaller radii than could be shown by this method. For the oxide film it appears by both techniques that the maiority of the pores are concentrated in the 30-50 A. radius range. With but one exception, the total pore volume, as measured
1419
by the n-butane system, was larger than the total pore volume as measured by the porosimeter in the range 45,000-15 A. This indicates that pressures higher than 60,000 p.s.i. would be required for the mercury to penetrate the extremely fine pores in the samples. It is concluded that the mercury porosimeter gives satisfactory pore si22 distribution information for pores as small as 30 A. in diameter when pressures up to 60,000 p.s.i. are employed. This technique gives some insight into the size and distribution of extremely small pores below the range measurable by n-butane sorption using presently known methods of calculations. A further increase in the working pressure range of a porosimeter to investigate even smaller pores seems desirable.
THERMODYNAMIC DATA I N DONOR-ACCEPTOR FORM’ BY HENRY A. BENT School of Chemistry, University of Minnesota, Minneapolis 1.6,Minnesota Received June i4
