Inorganic Oxide Air Adsorption at Room Temperature and Correlation with Surface Area JOHANNES TUUL and W. 6. INNES American Cyanamid Co., Sfamford, Conn.
b A gas pycnometer method of measuring air adsorption at room temperature is described which allows quick estimation of surface areas when area and adsorption are proportional. It has been applied to various inorganic oxide catalysts and appears to correlate well with the B.E.T. method. It is much less elaborate than the latter and particularly useful for routine work on catalysts. The proportionality constant should be determined for each type of material since it varies considerably with the chemical composition.
specimens during the measurement. Using known or “helium” skeletal densities for some materials, the amounts of air adsorption were calculated. Comparison of the adsorption values with surface areas obtained by the B.E.T. method revealed a positive correlation. This observation suggested the use of the pycnometer for estimation of surface areas. During this investigation, some interesting observations were made concerning t.he materials under study. If helium is used instead of air, the volume of the sample available to helium is measured. The difference between this volume and the measured volume with air can be attributed to adsorbed air. This difference in volume readings indicates the change in air adsorption on increasing the pressure from 1 to 2 atm. as can be shown by gas law considerations. As nitrogen and oxygen normally obey Henry’s law a t room temperature (2, 6), the measured adsorption is practically equivalent to the air adsorption a t 1atm. pressure.
T
HE use of the air comparison pycnometer for skeletal density measurement has been described elsewhere (8). An application which is quite different from that given by its manufacturer, Houston Instrument Corp., Houston, Tex., is discussed below. When the instrument was used in our laboratory for examining catalyst materials, obviously erroneous results were obtained and sometimes even negative volume readings were found. This was an unequivocal indication that a substantial amount of air was adsorbed on such
EXPERIMENTAL
Over 100 samples, most of them alumina base catalysts, were subjected to various heat treatments. Changes
Table 1.
Materials
Original CalcinaCompacted tion Bulk Tempera(11, ture, Meter2/ Density, c. Gram Gram/Cc. Surface Area
Notation
A AM AMC
AMN AP
S SA1
SA2 SA3
Composition Physical State Gamma-alumina l/ls-inch extrudates 90% A1203 10% Moos X a/l,-inch pellets
+
+ +
A1203 15% MOO8 f 3% COO 1/16-inchextrudates 82% A1203 15% MOOS 3% NiO l/la-inch extrudates Al2O3with 0.6% P t 1/16-inchextrudates Si02 microspheres 87% Si02 13% ALOa microspheres
82%
+ 85% Si02 + 13% microspheres 75% Sioz + 25% &Oa x a/16-inchpellets 1/8
818 *
ANALYTICAL CHEMISTRY
+
240
0.56
593
200
1.00
593
270
0.59
482
180
0.61
510
200
0.65
593
700
0.43
200
550
0.44
593
130
0.82
593
310
0.76
700
in weight and pycnometer reading with air, due to heat treatments, were recorded, and skeletal densities were determined with helium as described previously (8). From these data the adsorption of air was calculated as described above. Some samples were exposed to room air by leaving their containers open; others were kept in sealed containers; and still others were kept in a desiccator. Since no differences were observed in the behavior of the samples in the last two categories, the desiccator procedure was later abandoned. Room ternperature-ie., the temperature of the experimentvaried between 19’ and 27’ C., and relative humidity between 30 and 70%. Deviations from the average temperature (23’ C.) did not cause large variations in air adsorption (about l%/’ C.) and have therefore been neglected. For highest accuracy all observations should be carried out a t the same temperature a t which the method is calibrated. To determine whether the adsorption of air in the pycnometer was due to or affected by moisture in room air, a few measurements were carried out with dry air. For this purpose, the pycnometer was evacuated and filled with air which was dried by letting it pass through a Linde Type 4A Molecular Sieve filter. Observations were also made on samples of three different substances while exposed to air which was dried by passage through molecular sieve filters. To determine whether the adsorption was due to oxygen or nitrogen or both, some measurements were carried out with oxygen and nitrogen separately. The investigated materials are listed in Table I. INFLUENCE OF HEAT TREATMENT ON ADSORPTION
The influence of heat treatments and exposure to room air on gamma alumina was studied first. The weight of the sample decreased with increasing heat treatment temperature. Up to about 600’ C., there were corresponding changes in the pycnometer reading which indicated increasing amounts of adsorption (it should be noted that this adsorption was reversible-i.e., the adsorbate was desorbed when the pressure in the pycnometer was returned to atmospheric). At 800” C., however, the pycnometer reading changed in the opposite direction, thus indicating the on-
0 50-
$
I
1
I
1
400
I
500
600
700
800
ACTIV4TION TEMPERATURE ,'C
Figure 1. Room temperature air adsorptive capacity vs. activation temperature
set of a sintering process. After the last heat treatment the samples were first kept in sealed containers and exposed to room air only while measurements were carried out with them. The increases in weight were very small during that period. However, when the container of a sample was left open, the increases in weight and pycnometer reading were very rapid a t first, then gradually leveled off toward equilibrium values. Similar studies were carried out with several alumina base catalysts. Air adsorption on each material was studied as a function of the duration as well as the temperature of heat treatment. The duration of the heat treatment was varied from I5 minutes to 6 hours. I t was found that a prolongation of heating beyond 30 minutes had very little effect. The results (0.5 to 5 hours) are shown in Figure 1 for five materials. Each point represents the average of five tests on samples treated a t the given temperature. All curves have a Bat maximum in the neighborhood of 550" C. For that rcason the writers used treatment a t 550" C. as a standard prior to air adsorption measurements on alumina base catalysts. The increase in air adsorption with increasing temperature of treatment might be explained on the basis of dehydration and higher surface area. The decrease after the maximum is believed to be due to sintering with the accompanying loss of surface area (3, 7 ) .
adsorption between dry air and regular room air. Thus, the adsorption was due to either oxygen or nitrogen, or both, and small amounts of moisture in the measurement air had negligible effect. Measurements were then made with prepurified tank oxygen and nitrogen separately. More nitrogen than oxygen was adsorbed under these conditions. The amount of oxygen adsorbed was on the average about 87% of nitrogen adsorbed under same conditions. This was true for all A1203, M203-Mo03,and A1203-fiilo03-Co0 samples studied, and independent of the state of the sample-Le., independent of whether the sample had been freshly calcined or exposed to room air for any period of time. Using the nitrogen and oxygen adsorption data, we calculated the expected amount of adsorption assuming Henry's law for a mixture of 80% nitrogen and 20% oxygen. The calculated values were in excellent agreement with the experimental values for air adsorption. Lambert, Peel, and Heaven (4, 5) have made a similar observation with silica gel; they found that nitrogen and oxygen adsorb approximately in the ratio 1.0 to 0.9 a t 0" c. When a sample was exposed to room air after calcination, its weight increased whereas its air adsorptive capacity decreased. These changes were due to water vapor adsorption. This was
i
\
1 , Figure 2. Changes in air adsorptive capacity of sample of AlzO&toO;COO vs. weight gain due to adsorbed water
true with all materials studied. Results of an experiment of the kind used in this study are shown in Figure 2 where air adsorption is plotted us. per cent weight gain of sample No. 91. A control sample (No. 85) of the same material was exposed to room air only part of the time. During that period the changes in both samples were very similar. When the control sample was exposed to dry air, or its container was tightly sealed, its weight and air adsorptive capacity remained unchanged.
INFLUENCE OF MOISTURE ON WEIGHT AND AIR ADSORPTION
The question arose as to whether the adsorption of the room air used for pycnometer measurements was affected by its moisture content. To study this, the instrument was evacuated and dry air was admitted. Within experimental accuracy there was no difference in the
Figure 3.
Air adsorption "surface areas" vs. B.E.T. surface areas VOL. 34, NO. 7, JUNE 1962
819
Table II.
Observed Values of the Constant C
Notation A AM
Material A1203
90% Also3 A M C 82% A1203
+
+ 10% h50Oa + 15% MoOa
3% coo A M N 82% A1203 j-15% ?vloOa
+ 3% N10
S SA1 SA2
CY0
Meterz/Cc.
SiOz SiO2-AI2O3(13% AlzOs)
650 460 490
values were compared with "B.E.T. surface areas" (1). The average ratios between surface area and air adsorption obtained with various materials are given in Table 11. After being calibrated, the pycnometer can be used to estimate surface areas. The formula for this is A = CVa/W
490
where
700 790
A
SiO2-A12O3(25% &Oa) 750 a Average ratio of specific area to specific air adsorption.
SA3
From these and similar observations with samples of different materials, it was concluded that no changes occurred in the adsorptive capacity of the studied materials as long as moisture was excluded from them. The minute changes observed while the samples were kept in sealed containers may be attributed in part to moisture leaking into the containers, and in part to exposure to regular room air while carrying out observations on weight and air adsorption. CORRELATION OF AIR ADSORPTION WITH SURFACE AREA
Physical adsorption in the Henry's law range is proportional to both surface area and the Henry's law constant which is a measure of attraction of the surface for air. As most inorganic oxides have rather similar surfaces, it appeared likely that the Henry's law constant would be about the same for all and that, therefore, the surface area would be proportional to the air adsorption. To test this possibility, measured air adsorption
=
C(l/D
-
T'ai,/W)
surface area per gram of sample estimated from air adsorption V , = change in air adsorption when pressure is increased from 1 t o 2 atm. at ambient temperature (23" =t4" C.) W = sample weight in grams D = skeletal density (gram per cc.) determined using helium VBir = pyncometer reading Kith air (in cc.) C = a constant characteristic of the substrate =
V ois the difference between pycnometer readings (helium reading minus air reading). From the reading with helium, the skeletal density, D, can be calculated, and when this is known, the second form of the formula can be used. Since the proportionality constant, C, varies from one material to another, the method should be calibrated for each material individually. However, if not too high an accuracy is required, one proportionality constant may be used for a number of closely related materials. Thus, for the three molybdena-containing materials the maximum error when using C = 460 would be less than 7%. When C and D are known, routine measurements on the same substance can be carried out quickly. Less than 5 minutes are needed to obtain the weight and the pycnometer reading with air, and from these the air adsorption "surface area" per gram can be calculated.
For samples with air adsorption in excess of skeletal volume, pycnometer readings are negative and cannot be obtained by the normal procedure. Handwheel B (see operating instructions) can be backed off from the stop position 1, 2, or 3 turns, as required, to bring the pointer on the scale. With our unit, one turn is equivalent to 2.9 cc. Figure 3 shows the correlation of surface areas, calculated from air adsorption by the use of the constants in Table 11, with B.E.T. surface areas (1). REPRODUCIBILITY
Many check measurements were made a t constant temperature. Standard deviation in pycnometer reading averaged 0.025 cc. In the case of 200 sq. meters per gram samples with an alumina base, this produces a standard deviation in surface area of about 0.5%; for 50 sq. meters per gram, about 2%, etc. The lack of high precision a t low surface areas, which in lesser degree is shared by low temperature nitrogen methods, is not normally important for work on catalysts. LITERATURE CITED
(1) Innes, W. B., Ax.4~.C H m . 23, 759 (1951). ( 2 ) Kalberer, W., Schuster, C., 2. physik. Chem. A141,274 (1929). ( 3 ) Krieger, K. A., J . Am. Chem. SOC. 63,2712 (1941). (4) Lambert, B., Heaven, H. S., Proc. Roy. SOC.(London)A153, 584 (1936). (5) Lambert, B., Peel, D. H. P., Ibid., A144, 205 (1934). ( 6 ) Pvlagnus, A., Grahling, K., 2. physik. Chem. A145,27 (1929). ( 7 ) Milligan, L. H., J . Phys. Chem. 26, 247 (1922).
(8) Tuul, J., DeBaun, R. M., ANAL.
CHEM.3 4 , 8 1 4 (1962).
RECEIVEDfor review January 19, 1962. Accepted April 9, 1982.
FIuorosuIfonic Acid as Titrant in Acetic Acid RAM CHAND PAUL, SHAM KUMAR VASISHT, K. C. MALHOTRA, and SARVINDER SINGH PAHlL Deparfmenf of Chernisfry, Panjab University, Chandigarh-3, lndia Fluorosulfonic acid forms a solid compound HSOIF. CHKOOH with acetic acid. This compound has a high conductivity (2.4 X 10-2 ohm-' cm.-' at 60" C.). A comparison of the equivalent conductivities of various acids in acetic acid and comparative potentiometric titration studies of perchloric acid and fluorosulfonic acid in acetic acid indicate that in this medium fluorosulfonic acid is a slightly stronger acid than perchloric acid. Conductometric, potentiometric, and visual ti-
820
ANALYTICAL CHEMISTRY
trations using malachite green and crystal violet have established the suitability of fluorosulfonic acid as an acidic titrant in acetic acid.
A
is widely used as the medium for nonaqueous titrations. The acid-base reactions occurring in the solvent are well established ( 2 , 8, 9). The solvent system is represented as CETIC ACID
2CHaCOOH
CHaCOOH2+
+ CHaCOO-
In this medium, only strong protonic acids can act as acids. Organic bases and acetates behave like strong bases; even weakly basic compounds have their basicity enhanced (9). Perchloric acid has been considered the strongest acid in acetic acid. When perchloric acid is used in this medium, even weakly basic substances which cannot be estimated by direct titration in water, such as amines (6), sulfonamides ('i'),and alkaloids (IO,l'i'),have been successfully titrated. Mixtures of
