1332
INDUSTRIAL AND ENGINEERING CHEMISTRY
(2) Dubinin, & M., ‘I. and Radushkevich, L. V., Compt. rand. aced. S C ~ .U.R.S.S., 55, X O . 4, 327-9 (1947). (3) Dubinin, &M., !I.and Timofeyev. D. p., Zbid.9 54, No. 8, 701-4
(1946); 55, No. 2, 137-9 (1947). ( 4 ) Freundlich, H., “Colloid and Capillary Chemistry,” by H. S.
Hatfield from znd German ed., ~ 1926.
~Methuen ~ and dco,,
( 5 ) Harkins, W. D., and Jura, G., J . Chem. Phys., 11,430 (1943). ( 6 ) Langmuir, I., J . Am. Chem. Soc., 40, 1361 (1918). (7) Lewis, W. K., Gilliland, E. R., Chertow, B., and Bareis, D., Zbid., 72, 1160 (1950). ( 8 ) Lewis, $3’. K., Gilliland, E. R., Chertow, B., and Cadogan, W. P., I W D . EKG.C H E M . , 42, 1319 (1950).
Vol. 42, No. 7
(9)Lewis, W. K., Gilliland, E. R., Chertow, B., and Hoffman, W. H., J . Am. Chern. Soc., 72, 1163 (1950). (10) Lewis, Tv. K., Gilliland, E. R., Chertow, B., and Miliiken, w., Ibid., 72, 1157 (1950). ~ (11) PolanYi, ~ , hf.7 Verhandt. deut. p h w . k . Ges., 16, 1012 (1914); 18, 65 (1916); 2. Elektiochem., 26, 370 (1920)., (12) Reyerson, L. H., and Cines, >I. R., J . Phus Chem., 46, 1060 (1942). (13) Taylor, H. S . , and Bomlla, C. F., 1x11.EXG.CHLM.,39, 871 (1947).
RECEIVED December 5 , 1949.
Alumina Surface Area Measurements By Nitrogen, n-Butane, Propane, a n d S t e a r i c Acid Sorption ALLEN S. RUSSELL AND C. NORMAN COCHRAN, Aluminum Company of
America, New Kensington, Pa.
Surface areas calculated by the method of Brunauer, E m m e t t , and Teller (BET)have been compared by n-butane and nitrogen sorption for aluminas of varying pore width, crystal structure, and surface composition. For most of these materials a rather consistent correlation was found, leading to a n n-butane molecular area of 33 A.2 on the basis of 16.2 A.2 for nitrogen. Comparison of n-butane and propane sorption has shown t h e same number of molecules of each to be held in a BET monolayer. The surface a r e a of nonporous hydrated aluminas measured by stearic acid sorption from solution agree with those by nitrogen if the stearic acid molecular area is 17 A.2
HE useful Brunauer-Emmett-Teller ( 8 ) (BET) equation for surface area measurements can in principle be applied equally well t o the sorption of any nonreactive gas. However, there is now general agreement that routine surface areas should be standardized by nitrogen sorption, in view of the wide experience with this gas and the lack of anomalies occasionally observed with other systems. Nonetheless, it has frequently proved advantageous to determine B E T areas with materials other than nitrogen, and more work of this type is needed to provide a broader experience for improving the theory-. n-Butane has been employed a t these laboratories for relative surface area estimations (IO)because of its similarity to hydrocarbons of interest in catalysis and because it is sorbed to a convenient extent a t ice temperature and hence eliminates the use of liquid nitrogen baths. The areas have been expressed in the experimental units, millimoles of n-butane sorbed in a BET monolayer per gram of sample (m.b./g.). It is convenient to convert this unit t o square meters per gram on the nitrogen basis, and this can be done if the relative molecular areas of nitrogen and nbutane on alumina are known. Emmett and Brunauer ( 4 )originally assigned a molecular area of 32 A.2 to n-butane from the geometric formula for the area of close-packed spheres and the liquid densities by which they selected 16.2 11.2 for nitrogen. Later workers (9, 5 , 8 )have found 32 A.2 to be too low, but their values have ranged from 41 to 56.6 A.2 I n this paper n-butane and nitrogen areas are compared for several samples of alumina, a material of particular interest because of the variety of structures available. I n searching for the factors influencing the molecular areas on an alumina surface, B E T monolayers were calculated from both n-butane and propane sorption. Because the results give a clearer picture of the orientation of the n-butane molecule on an alumina surface, typical data are presented here. Comparative areas are also shown for finely divided, nonporaus aluminas which
demonstrate a consistent correlation between stearic acid and nitrogen sorption. However, the molecular area of stearic acid by comparison with nitrogen does not agree exactly with values from crystallographic measurements. GAS SORPTION
The gas sorption systems were of conventional volumetric type. Sample bulbs with interchangeable tapered joiut,s were employed, which were lubricated by either dpiezon L or silicone grease. The n-butane and propane were the 99% gases of the Ohio Chemical Company. These were purified by sorbing a quantity onto activated alumina in a storage bulb and partially evacuating the residual gases. The n-butane and propane contained lesa than 0.1% noncondensable impurity as determined by test before use. Kitrogen was the prepurified (99.9%) product of the Matheson Chemical Company passed over hot copper a t 500” C. and through a liquid nitrogen trap. For n-butane and propane sorption the samples were held a t constant temperature by a bath of crushed ice. For nitrogen sorption the bath was liquid nitrogen, the temperat,ure of which was measured with a nitrogen vapor pressure thermomet,er. The samples were usually evacuated by a mercury pump withmm.) for 1 hour a t 140” C. before out a cold trap (pressure measurement. The surface areas did not depend critically on this evacuation unless a considerable amount of water had been sorbed on the alumina. In this case longer times or higher temperatures of evacuation were employed. Two-gram samples were used for n-butane and propane sorption, while 1-gram samples were used for nitrogen sorption. The bulb free spaces a t temperature were 20 ml. for the n-butane system and 6 ml. for the nitrogen system. For comparative measurements the samples were crushed t o about 30-mesh and carefully, sampled. The very finely powdered samples were tableted wlthout a binder and then crushed before measurement. Equilibrium times to give sufficient constancy of pressure that calculated areas were reproc3ucible t o 1% were about 10 minutes for both s y s t e m , and appeared t o be governed by heat flow. Corrections were made in the calculations for deviations from the perfect gas laws. Usually four or five sorption points were taken for a surface area determination.
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
July 1950
b
a
n
0
0.10
0.20 0.30
0
0.20
0.10
P
0.30
P
G
p7,
Y
0.10
lower sorption of impurities in the room temperature part of the system, and the freedom from gas imperfection corrections a t room temperature. On the other hand, n-butane is preferable from the standpoints of constant temperature of sorption, smaller free space correction in the sorption bulb (because of its higher temperature), and elimination of the liquid nitrogen bath. Nitrogen is much to be preferred for use in a one-point estimate of surface area, because this requires that the B E T c value be high. The higher c value for nitrogen (which is related to the lower sorption temperature) also appears to be responsible for the increased linearity of the B E T plot compared to n-butane sorption. Nitrogen shows its greatest superioritj. for alumina samples of very low c value with n-butane. There is little apparent advantage in the use of propane at 0' C. over n-butane unless it is desired to examine the region of low relative pressure (sorption pressure divided by condensation pressure). The correction for gas in the free space of the bulb is larger with propane than with n-butane. The gravimetric stearic acid procedure is limited t o nonporous or wide-pore materials. For fine particle, nonporous alumina it is more rapid than the conventional gas sorption procedure when many samples are t o be measured, and it is well adapted to use by personnel unaccustomed to vacuum techniques.
:l/I
( L o
0
1333
0.20
0.30
0
0.20
010
0.30
P -
P
K
PO
d
C
Figure 1. Types of BET Curves STEARIC ACID SORPTION Previous workers (6, 9) have taken great care t o degas their samples before exposing them to the stearic acid solution. For these nonporous materials the same reproducibility can be achieved by a simple drying. Although the benzene used in the measurements was dried by activated alumina before use, a small quantity of water was found to have little influence on the results. The benzene was the C.P. product of the Baker and Adamson Company, and the stearic acid was the C.P. Eimer and Amend powder. Five-gram samples of alumina in 50-ml. glass-stoppered flasks were dried to constant weight a t 115" C. Fifty milliliters of 0.05 molar stearic acid in benzene at 25" C. were weighed into the flask and held a t 25 O C. for 1 hour with intermittent shaking. After settling for 3 hours in the thermostat, 10 to 15 ml. of the clear solution were pipetted carefully into% weighing bottle. The clear solution was weighed, after which the benzene was evaporated slowly in a hood a t 85 C. until the weight loss was less than 0.0005 gram in successive 1-hour periods. The weight of stearic acid was increased 0.1% for each hour the sample was heated, to compensate for stearic acid loss. The initial stearic acid concentration was established by a blank run. The amount of stearic acid sorbed was calculated from the formula : O
Millimoles of stearic acid sorbed = grams of alumina
where A = weight of dried alumina T weight of stearic acid solution added t o alumina S = solution weight withdrawn for evaporation & = solution weight withdrawn for evaporation of = blank E = evaporated weight of S Eb = evaporated weight of Sb
APPLICATION OF BET THEORY TO n-BUTANE AND PROPANE SORPTION In a number of cases the B E T plot for n-butane or propane sorption on alumina is not linear even over a relatively small pressure range. Four general types of B E T plots have been encountered which can be correlated roughly with the B E T c value. The c value for n-butane sorption is expressed in terms of the BET quantity &-EL, "heat of sorption minus heat of liquefaction." E1 -EL is calculated from the product 1.25 log c for sorption a t 0" C. The first type of B E T plot (Figure 1, a)is linear from a relative pressure of 0.01 to 0.20, and the experimental points lie above the line a t higher pressures. This type is found for high E 1 - E ~ values, usually over 1.85. As EL-EL is lowered t o about 1.75 the experimental B E T points a t relative pressures up to 0.05 fall below the linear plot, while points a t relative pressure greater than 0.20 approach the line more closely. With further decrease to about 1.5, the experimental points a t relative pressures near 0.3 agproach the linear plot (Figure 1, c). For values below 1.4 the experimental points a t 0.2 relative pressure fall below the B E T line, forming a curve whose slope is continuously diminishing as pressure incremes (Figure 1, d ) . A similar system displaced to lower E1 -E L values is found for propane, The correlation of E1 -EL values with type of B E T plot is imperfect. The lack of linearity of the B E T plot makes some judgment necessary in the application of n-butane to surface area measurements. The authors' technique is t o draw a straight line through the B E T curve a t 5-cm. and IO-cm. pressure (about 0.065 and 0.13 relative pressure), because this gives the most consistent relations between BET areas. A principal advantage of nitrogen sorption is that straight lines over the 0.05 t o 0.20 relative pressure range are the general rule for the samples of this investigation.
=I
CHOICE OF METHOD FOR MEASURING AREAS There is little t o choose in ease of experimental manipulation between nitrogen and n-butane sorption. Nitrogen sorption gains convenience from the lesser effect of stopcock grease, the
CALCULATION OF SURFACE AREAS ACID SORPTION
FROM STEARIC
The stearic acid technique, of course, depends on sorbing an exact monolayer from the solution. This condition might be expected t o apply only if the concentration of the solution is carefully selected. However, in accord with the work of Saunders (11) and of Harkins and Gans ( 7 ) , the conditions for obtaining constancy of sorption, and hence presumably monolayer sorp-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1334
Table I. Alumina Surface Areas from Nitrogen and n-Butane Sorption Nitrogen Area, Sq. 3Ieters/G.
n-Butane Area, E1 - EL, kg.-cal.
m.b.jg.
n-Butane Molecular Alea, .4.2
Heated a-Trihydmte
Alcoa A-1 1 hour 300' C. air 1 hour 400' C. air
Heated 8-Trihydrate 354 1.59 280 1.30
1.99 1.78
37.0 35.8
Alcoa C730 Alcoa D 5 0
Hydrated Aluminasa 21.8 0.096 22.4 0,095
1.1 1.0
38 39
Alcoa H41 Alooa H41
Heated Gel .4luminas 322 1.35 383 1.68
1.41 1.29
39.6 37.8
1.43 1.45 1.81
39.7 40.3 39.3
Impregnated Aluminas 179 0 75 Alcoa F l 5% Si02 Alcoa F1 5% Si02 172 0 71 Alcoa F10 hfo/.41 = 0.05 101 0 43 " Evacuated 1 hour a t 100' C.
tion, are readilj- achieved. This is further shown by the folloa-ing data: Temperature,
c.
25 25 25 25 50
Initial Stearic Acid Molarity 0,025 0.050
0.050 0.100
0.050
Final Stearic Acid Molarity 0,010 0.012 0.018 0,069 0,021
&lillimole Stearic Acid Sorbed/Gram Alumina 0.47 0.42
0.46 0.48 0.42
COMPARISON OF n-BUTANE AND NITROGEN AREAS In Table I are shown comparative surface areas from sorption of nitrogen and n-butane on various aluminas. The columns give the description of the sample, the surface area in square meters per gram calculated by nitrogen sorption assuming the nitrogen molecular area to be 16.2 A . 2 , the surface area from nbutane sorption expressed as millimoles of n-butane per gram, and t,he E1-Eh value for n-butane sorption. The final column of the table gives the nitrogen and n-butane sorption results expressed as molecular area of n-butane in A.* This is calculated from the nitrogen area (square meters per gram) divided by n-butane area
Figure 2. Orientation of Sorbed n-Butane and Propane Molecules Nitrogen in upper right f o r size comparison
Vol. 42, No. 7
(m.b./g.) and divided by 6.02. BET c values for nitrogen are not given because they are uniformly high (over 50), of low precision, and apparently unrelated to the alumina structure. The first comparison is for a series of aluminas prepared by heating various samples of a-alumina trihydrate precipitated from sodium aluminate solution. -4s t'he alumina trihydrate is heated, water is driven off and the alumina transforms first to a monohydrate and then to a succession of nearly anhydrous crysballine forms. Because these materials are all more dense t'han the trihydrate and the particles do not shrink appreciably during this heating, many small cracks and crevices develop and a structure of high surface area and small pores is created. As heating is continued above about 400" C., crystal growth sets in, so that the surface area diminishes and the pore widths (calculated by complete n-butane sorption isotherms not shown here) become greater. It might be anticipated that the area available to nitrogen would be relatively greater than for n-butane for the first samples in the sequence and that the difference would diminish as the pores widen. The results of Table I do not show this effect to an appreciable extent but rat'her show a constant ratio between n-butane and nitrogen areas equivalent to an n-butane molecular area of about 39 A,* There is some unexplained scatter t o the data. The activation of 6-trihydrate proceeds much like that of atrihydrate, and the effective n-butane niolecular area is roughly the same. The hydrated aluminas were initially produced in particles so tiny that they had appreciable surface area on their external geometric surfaces. These materials are nonporous or at least have only very wide pores, so that the area should be equally available to the n-butane and to the nitrogen. Thus these aluminas should be particularly suited for comparison of areas calculated by nbutane and nitrogen sorption. Unfortunately, the n-butane areas for these samples are of low precision because of an unusually low E, - E L value which makes the BET theory inaccurate. The heated gelatinous aluminas have average pore width similar to that of a-trihydrate heated past the maximum to an area of about 100 square meters per gram. The n-butane molecular area for heated alumina gels containing about 57, silica, -4lcoa H41, was about like t'hat for the heated a-t'rihydrates. Silica impregnated onto heated a-trihydrate has no significant effect on the n-butane molecular area.
Table 11. Surface Argas of Coked Catalysts from Nitrogen and n-Butane Sorption (Samples evacuated 1 hour a t 25' C . ) C,uke, IC
Nitrogen Area, Sq. Ivleters/G
Area, m.b./g.
n-Butane El EL, kg.-oal.
-
n-Butane Molecular Area, A.2
The coniparison of areas for alumina impregnated Kith molybdena is pertinent because of the importance of these materials in catalj-tic dehydrocyclization. The n-butane area for a sample of a-trihydrate impregnated after activation with ammonium molybdate to an atomic ratio of molybdenum to aluminum of 0.05 and heated 1 hour at 500' C. gave the same n-butane molecular area as for the unimpregnated alumina. In Table I1 are shown surface areas calculated from nitrogen and n-butane sorption for silica-impregnated alumina catalysts on which coke has been deposited during catalytic cracking. Although the deposited coke undoubtedly changes the pore size distribution of the sample, the n-butane molecular area is like that,
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
July 1950
1335
Two other comparisons of surface areas determined b y ,fatty acid and nitrogen adsorption are reported in the literature. Smith and Fusek (19) n-Butane Propane found agreement between the two methods for Area, E1 - EL Area, I 1 - EL, Propane Raney nickel catalysts when the area of the m.b./g. kg.-Gal. m.p./g. kg.-cal. Molecule Butane palmitic acid molecule was assumed 20.5 A.2 in a-Trihydrate in one case and 16.0 A.2 in the other with nitrogen 5 min. 350' C. air 0.31 2.12 0.30 1.84 0.97 0.41 2.05 0.48 1.63 1.17 a t 16.2 A.2 Ries, Johnson, and Melik (9) deter'5min. 350' C. air 1.05 1.83 1.01 1.82 0.96 16 min. 380' C. air mined the areas of various porous catalysts by 0.94 6 min. 400" C.air 1.09 1.57 1.02 1.52 15 min. 400° C. air 1.09 1.61 1.11 1.42 0.96 1.02 nitrogen and stearic acid. For one sample they 1.23 1.67 1.18 1.53 5 min. 430° C. air 1.24 1.45 1 .oo found a stearic acid area of 19.5 A.2 to be consistent 2 hours 400' C.air 1.24 1.54 1.15 1.34 1.16 1.21 1.01 Alcoa F1 with nitrogen a t 16.2 A.2 Their other results are 1.04 0.82 1.34 0.85 1.24 Alcoa F1 1.07 0.50 1.60 0.81 1.47 Alcoa F10 not suitable for a comparison of molecular areas, 2 hours 450 steam 0.54 1.47 0.00 1.14 1.11 Alcoa H40 R2402 0.90 1.13 0.85 1.24 0.94 because the porosity obviously interfered with sorpAleoa F10 Mo/A1 = tion of stearic acid on a portion of the area. I n 0.05 0.39 1.73 0.42 1.63 1 .os the authors' work, the stearic acid technique with a porous alumina showed an apparent surface area of Table IV. H y d r a t e d A l u m i n a Surface Areas f r o m 52 square meters per gram (stearic acid molecular Nitrogen a n d S t e a r i c Acid S o r p t i o n area 16.7 A.2) for a sample with a surface area of 110 square Stearic Acid Nitrogen Millimoles Nitrogen meters per gram by nitrogen. Area Area T a b l e 111. A l u m i n a S u r f a c e Areas f r o m n - B u t a n e a n d P r o p a n e Sorption
I
Millimolk/G. 0.195 0.225 0.177 0.121 0.099 0.099 0.098 0.091 0,085
Millimol;/G. 0.217 0.214 0.200 0.130 0.112 0.104 0.094 0.094
0.092 0.090 0.089 0,080 0.076 0.071
_
_
0.083 0.093 0.085 0.072
0.068
Millimoles Stearic Acid 1.11 0.95 1.13 1.07 1.02 1.05
0.96 1.03 1.03 1.09 0.96 0.94 1.05 1.04
~
for the regular alumina samples, It might have been thought that after coking the smaller nitrogen molecule would be able t o penetrate pores that were not available t o the n-butane. COMPARISON OF n-BUTANE AND PROPANE AREAS I n Table I11 the B E T surface areas are compared from nbutane and propane sorption a t 0 ' C . for calcined e-trihydrate samples, a heated gelatinous alumina, and a molybdena-impregnated alumina, The propane sorption results are expressed EM millimoles of propane in a B E T monolayer per gram of sample (m.p./g.). The last column of the table shows a 1 t o 1 ratio for the molecules of propane in a B E T monolayer per molecule of nbutane in the B E T monolayer. The &-EL value is uniformly lower for propane than for n-butane sorption. It was expected that more of the smaller propane molecules than butane would be adsorbed in a monolayer. The 1 t o 1 ratio is interpreted t o mean that the n-butane molecule is oriented on the surface like propane, with the extra methyl group raised from the surface enough to fit over the first methyl group of the next butane molecule as shown in Figure 2. The correct area of the nbutane molecule from this geometry should be about 39 A.2if the nitrogen molecular area is taken as 16.2 A.2 A model of the nitrogen molecule is included in Figure 2 to show its comparative size. COMPARISON OF NITROGEN AND STEARIC ACID AREAS In Table I V are shown BET areas calculated from nitrogen and stearic acid sorption for a series of fine-particle a-alumina trihydrates, Alcoa hydrated aluminas C730. The various surface areas in the C730 aluminas a ere obtained by differences in precipitation procedure. The linear correlation between the two sets of results over a threefold range of area gives 16.7 A.2 for the area of the stearic acid molecule when the area of the nitrogen molecule is assumed to be 16.2 A.2 If the stearic acid area is actually 18.4 A.2 as calculated from x-ray data ( I ), then the nitrogen molecular area is calculated as 17.8 A.2 If the adsorbent were slightly porous, high rather than low values for the stearic acid molecular area would result.
CONCLUSIONS Alumina surface areas can be compared reliably by application of the B E T theory t o n-butane sorption. The n-butane molecular area is about 39 A.a calculated from comparative B E T monolayers for a range of samples. These include heated or-alumina trihydrates, heated alumina gels, molybdena-impregnated alumina dehydrocyclization catalysts, silica-alumina cracking catalysts, and silica-aluminas with deposited coke from catalytic cracking. These samples differ widely in pore widths and crystal structures. The consistent agreement in the measured samples between nbutane and nitrogen sorption was unexpected, inasmuch as a number of narrow pores might be formed in the activation of the hydrated aluminas and in the coke deposition. The molecular areas of n-butane and propane were essentially identical for a series of alumina samples. The geometric area of the propane, and hence of the n-butane with its end methyl group raised, agrees well with surface coverage deduced from molecular models. The surface areas of finely divided nonporous aluminas can be measured consistently by sorption of stearic acid from solution in a simple procedure. From the constancy of the sorption with change in stearic acid concentration and sorption temperature, it appears that a monolayer is held strongly t o the surface. Comparison with nitrogen sorption shows that the effective area of the stearic acid is 17 A.S if the nitrogen molecule area is 16.2 A.2 An area of 18.4 A.2 for stearic acid is indicated by crystallographic data. LITERATURE CITED (1) Adam, N. K., "Physics and Chemistry of Surfaces," p. 52,
London, Oxford University Press, 1930. (2) Brunauer, S., Emmett, P. H., and Teller, E., J . Am. Chem. SOC., 60, 309-19 (1938). (3) Davis, R. T., DeWitt, T. W., and Emmett, P. H., J. Phus. Colloid Chem.,51, 1232 (1947). (4) Emmett, P. H., and Brunauer, S., J . Am. Chem. Soc., 59, 1559 (1937). (5) Emmett, P. H., and Cines, M., J . Phys. Colloid Chem., 51, 1248, 1329 (1947). (6) Ewing, W. W., J . Am. Chem. SOC.,61, 1317-20 (1939). (7) Harkins, W. D., and Gans, D. M., J . Phys. Chem., 36, 86 (1932). ( 8 ) Jura, G., and Harkins, W. D., J . Am. Chem. Soc., 66, 1306-73 (1944). (9) Ries, H. E., Jr., Johnson, M. F. L., and Melik, J. S., J . Phys. Colloid Chem., 53, 638-60 (1949). (10) Russell, A. S., and Cochran, C . N., IND. ENG.CKEY.,42, 1336 (1950). (11) Saunders, L., J . Chem. SOC.,1948,969-73. (12) Smith, H. A., and Fusek, J. F., J. Am. Chem. SOC.,68, 229-31 (1946). RECEIVED
June 17.1949.
